As preparation for multi-target, this patch makes each inferior have
its own thread list.

This isn't absolutely necessary for multi-target, but simplifies
things.  It originally stemmed from the desire to eliminate the
init_thread_list calls sprinkled around, plus it makes it more
efficient to iterate over threads of a given inferior (no need to
always iterate over threads of all inferiors).

We still need to iterate over threads of all inferiors in a number of
places, which means we'd need adjust the ALL_THREADS /
ALL_NON_EXITED_THREADS macros.  However, naively tweaking those macros
to have an extra for loop, like:

     #define ALL_THREADS (thr, inf) \
       for (inf = inferior_list; inf; inf = inf->next) \
	 for (thr = inf->thread_list; thr; thr = thr->next)

causes problems with code that does "break" or "continue" within the
ALL_THREADS loop body.  Plus, we need to declare the extra "inf" local
variable in order to pass it as temporary variable to ALL_THREADS
(etc.)

It gets even trickier when we consider extending the macros to filter
out threads matching a ptid_t and a target.  The macros become tricker
to read/write.  Been there.

An alternative (which was my next attempt), is to replace the
ALL_THREADS etc. iteration style with for_each_all_threads,
for_each_non_exited_threads, etc. functions which would take a
callback as parameter, which would usually be passed a lambda.
However, I did not find that satisfactory at all, because the
resulting code ends up a little less natural / more noisy to read,
write and debug/step-through (due to use of lambdas), and in many
places where we use "continue;" to skip to the next thread now need to
use "return;".  (I ran into hard to debug bugs caused by a
continue/return confusion.)

I.e., before:

    ALL_NON_EXITED_THREADS (tp)
      {
	if (tp->not_what_I_want)
	  continue;
	// do something
      }

would turn into:

    for_each_non_exited_thread ([&] (thread_info *tp)
      {
	if (tp->not_what_I_want)
	  return;
	// do something
      });

Lastly, the solution I settled with was to replace the ALL_THREADS /
ALL_NON_EXITED_THREADS / ALL_INFERIORS macros with (C++20-like) ranges
and iterators, such that you can instead naturaly iterate over
threads/inferiors using range-for, like e.g,.:

   // all threads, including THREAD_EXITED threads.
   for (thread_info *tp : all_threads ())
     { .... }

   // all non-exited threads.
   for (thread_info *tp : all_non_exited_threads ())
     { .... }

   // all non-exited threads of INF inferior.
   for (thread_info *tp : inf->non_exited_threads ())
     { .... }

The all_non_exited_threads() function takes an optional filter ptid_t as
parameter, which is quite convenient when we need to iterate over
threads matching that filter.  See e.g., how the
set_executing/set_stop_requested/finish_thread_state etc. functions in
thread.c end up being simplified.

Most of the patch thus is about adding the infrustructure for allowing
the above.  Later on when we get to actual multi-target, these
functions/ranges/iterators will gain a "target_ops *" parameter so
that e.g., we can iterate over all threads of a given target that
match a given filter ptid_t.

The only entry points users needs to be aware of are the
all_threads/all_non_exited_threads etc. functions seen above.  Thus,
those functions are declared in gdbthread.h/inferior.h.  The actual
iterators/ranges are mainly "internals" and thus are put out of view
in the new thread-iter.h/thread-iter.c/inferior-iter.h files.  That
keeps the gdbthread.h/inferior.h headers quite a bit more readable.

A common/safe-iterator.h header is added which adds a template that
can be used to build "safe" iterators, which are forward iterators
that can be used to replace the ALL_THREADS_SAFE macro and other
instances of the same idiom in future.

There's a little bit of shuffling of code between
gdbthread.h/thread.c/inferior.h in the patch.  That is necessary in
order to avoid circular dependencies between the
gdbthread.h/inferior.h headers.

As for the init_thread_list calls sprinkled around, they're all
eliminated by this patch, and a new, central call is added to
inferior_appeared.  Note how also related to that, there's a call to
init_wait_for_inferior in remote.c that is eliminated.
init_wait_for_inferior is currently responsible for discarding skipped
inline frames, which had to be moved elsewhere.  Given that nowadays
we always have a thread even for single-threaded processes, the
natural place is to delete a frame's inline frame info when we delete
the thread.  I.e., from clear_thread_inferior_resources.

gdb/ChangeLog:
2018-11-22  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMMON_SFILES): Add thread-iter.c.
	* breakpoint.c (breakpoints_should_be_inserted_now): Replace
	ALL_NON_EXITED_THREADS with all_non_exited_threads.
	(print_one_breakpoint_location): Replace ALL_INFERIORS with
	all_inferiors.
	* bsd-kvm.c: Include inferior.h.
	* btrace.c (btrace_free_objfile): Replace ALL_NON_EXITED_THREADS
	with all_non_exited_threads.
	* common/filtered-iterator.h: New.
	* common/safe-iterator.h: New.
	* corelow.c (core_target_open): Don't call init_thread_list here.
	* darwin-nat.c (thread_info_from_private_thread_info): Replace
	ALL_THREADS with all_threads.
	* fbsd-nat.c (fbsd_nat_target::resume): Replace
	ALL_NON_EXITED_THREADS with inf->non_exited_threads.
	* fbsd-tdep.c (fbsd_make_corefile_notes): Replace
	ALL_NON_EXITED_THREADS with inf->non_exited_threads.
	* fork-child.c (postfork_hook): Don't call init_thread_list here.
	* gdbarch-selftests.c (register_to_value_test): Adjust.
	* gdbthread.h: Don't include "inferior.h" here.
	(struct inferior): Forward declare.
	(enum step_over_calls_kind): Moved here from inferior.h.
	(thread_info::deletable): Definition moved to thread.c.
	(find_thread_ptid (inferior *, ptid_t)): Declare.
	(ALL_THREADS, ALL_THREADS_BY_INFERIOR, ALL_THREADS_SAFE): Delete.
	Include "thread-iter.h".
	(all_threads, all_non_exited_threads, all_threads_safe): New.
	(any_thread_p): Declare.
	(thread_list): Delete.
	* infcmd.c (signal_command): Replace ALL_NON_EXITED_THREADS with
	all_non_exited_threads.
	(proceed_after_attach_callback): Delete.
	(proceed_after_attach): Take an inferior pointer instead of an
	integer PID.  Adjust to use range-for.
	(attach_post_wait): Pass down inferior pointer instead of pid.
	Use range-for instead of ALL_NON_EXITED_THREADS.
	(detach_command): Remove init_thread_list call.
	* inferior-iter.h: New.
	* inferior.c (struct delete_thread_of_inferior_arg): Delete.
	(delete_thread_of_inferior): Delete.
	(delete_inferior, exit_inferior_1): Use range-for with
	inf->threads_safe() instead of iterate_over_threads.
	(inferior_appeared): Call init_thread_list here.
	(discard_all_inferiors): Use all_non_exited_inferiors.
	(find_inferior_id, find_inferior_pid): Use all_inferiors.
	(iterate_over_inferiors): Use all_inferiors_safe.
	(have_inferiors, number_of_live_inferiors): Use
	all_non_exited_inferiors.
	(number_of_inferiors): Use all_inferiors and std::distance.
	(print_inferior): Use all_inferiors.
	* inferior.h: Include gdbthread.h.
	(enum step_over_calls_kind): Moved to gdbthread.h.
	(struct inferior) <thread_list>: New field.
	<threads, non_exited_threads, threads_safe>: New methods.
	(ALL_INFERIORS): Delete.
	Include "inferior-iter.h".
	(ALL_NON_EXITED_INFERIORS): Delete.
	(all_inferiors_safe, all_inferiors, all_non_exited_inferiors): New
	functions.
	* inflow.c (child_interrupt, child_pass_ctrlc): Replace
	ALL_NON_EXITED_THREADS with all_non_exited_threads.
	* infrun.c (follow_exec): Use all_threads_safe.
	(clear_proceed_status, proceed): Use all_non_exited_threads.
	(init_wait_for_inferior): Don't clear inline frame state here.
	(infrun_thread_stop_requested, for_each_just_stopped_thread): Use
	all_threads instead of ALL_NON_EXITED_THREADS.
	(random_pending_event_thread): Use all_non_exited_threads instead
	of ALL_NON_EXITED_THREADS.  Use a lambda for repeated code.
	(clean_up_just_stopped_threads_fsms): Use all_non_exited_threads
	instead of ALL_NON_EXITED_THREADS.
	(handle_no_resumed): Use all_non_exited_threads instead of
	ALL_NON_EXITED_THREADS.  Use all_inferiors instead of
	ALL_INFERIORS.
	(restart_threads, switch_back_to_stepped_thread): Use
	all_non_exited_threads instead of ALL_NON_EXITED_THREADS.
	* linux-nat.c (check_zombie_leaders): Replace ALL_INFERIORS with
	all_inferiors.
	(kill_unfollowed_fork_children): Use inf->non_exited_threads
	instead of ALL_NON_EXITED_THREADS.
	* linux-tdep.c (linux_make_corefile_notes): Use
	inf->non_exited_threads instead of ALL_NON_EXITED_THREADS.
	* linux-thread-db.c (thread_db_target::update_thread_list):
	Replace ALL_INFERIORS with all_inferiors.
	(thread_db_target::thread_handle_to_thread_info): Use
	inf->non_exited_threads instead of ALL_NON_EXITED_THREADS.
	* mi/mi-interp.c (multiple_inferiors_p): New.
	(mi_on_resume_1): Simplify using all_non_exited_threads and
	multiple_inferiors_p.
	* mi/mi-main.c (mi_cmd_thread_list_ids): Use all_non_exited_threads
	instead of ALL_NON_EXITED_THREADS.
	* nto-procfs.c (nto_procfs_target::open): Don't call
	init_thread_list here.
	* record-btrace.c (record_btrace_target_open)
	(record_btrace_target::stop_recording)
	(record_btrace_target::close)
	(record_btrace_target::record_is_replaying)
	(record_btrace_target::resume, record_btrace_target::wait)
	(record_btrace_target::record_stop_replaying): Use
	all_non_exited_threads instead of ALL_NON_EXITED_THREADS.
	* record-full.c (record_full_wait_1): Use all_non_exited_threads
	instead of ALL_NON_EXITED_THREADS.
	* regcache.c (cooked_read_test): Remove reference to global
	thread_list.
	* remote-sim.c (gdbsim_target::create_inferior): Don't call
	init_thread_list here.
	* remote.c (remote_target::update_thread_list): Use
	all_threads_safe instead of ALL_NON_EXITED_THREADS.
	(remote_target::process_initial_stop_replies): Replace
	ALL_INFERIORS with all_non_exited_inferiors and use
	all_non_exited_threads instead of ALL_NON_EXITED_THREADS.
	(remote_target::open_1): Don't call init_thread_list here.
	(remote_target::append_pending_thread_resumptions)
	(remote_target::remote_resume_with_hc): Use all_non_exited_threads
	instead of ALL_NON_EXITED_THREADS.
	(remote_target::commit_resume)
	(remote_target::remove_new_fork_children): Replace ALL_INFERIORS
	with all_non_exited_inferiors and use all_non_exited_threads
	instead of ALL_NON_EXITED_THREADS.
	(remote_target::kill_new_fork_children): Use
	all_non_exited_threads instead of ALL_NON_EXITED_THREADS.  Remove
	init_thread_list and init_wait_for_inferior calls.
	(remote_target::remote_btrace_maybe_reopen)
	(remote_target::thread_handle_to_thread_info): Use
	all_non_exited_threads instead of ALL_NON_EXITED_THREADS.
	* target.c (target_terminal::restore_inferior)
	(target_terminal_is_ours_kind): Replace ALL_INFERIORS with
	all_non_exited_inferiors.
	* thread-iter.c: New file.
	* thread-iter.h: New file.
	* thread.c: Include "inline-frame.h".
	(thread_list): Delete.
	(clear_thread_inferior_resources): Call clear_inline_frame_state.
	(init_thread_list): Use all_threads_safe instead of
	ALL_THREADS_SAFE.  Adjust to per-inferior thread lists.
	(new_thread): Adjust to per-inferior thread lists.
	(add_thread_silent): Pass inferior to find_thread_ptid.
	(thread_info::deletable): New, moved from the header.
	(delete_thread_1): Adjust to per-inferior thread lists.
	(find_thread_global_id): Use inf->threads().
	(find_thread_ptid): Use find_inferior_ptid and pass inferior to
	find_thread_ptid.
	(find_thread_ptid(inferior*, ptid_t)): New overload.
	(iterate_over_threads): Use all_threads_safe.
	(any_thread_p): New.
	(thread_count): Use all_threads and std::distance.
	(live_threads_count): Use all_non_exited_threads and
	std::distance.
	(valid_global_thread_id): Use all_threads.
	(in_thread_list): Use find_thread_ptid.
	(first_thread_of_inferior): Adjust to per-inferior thread lists.
	(any_thread_of_inferior, any_live_thread_of_inferior): Use
	inf->non_exited_threads().
	(prune_threads, delete_exited_threads): Use all_threads_safe.
	(thread_change_ptid): Pass inferior pointer to find_thread_ptid.
	(set_resumed, set_running): Use all_non_exited_threads.
	(is_thread_state, is_stopped, is_exited, is_running)
	(is_executing): Delete.
	(set_executing, set_stop_requested, finish_thread_state): Use
	all_non_exited_threads.
	(print_thread_info_1): Use all_inferiors and all_threads.
	(thread_apply_all_command): Use all_non_exited_threads.
	(thread_find_command): Use all_threads.
	(update_threads_executing): Use all_non_exited_threads.
	* tid-parse.c (parse_thread_id): Use inf->threads.
	* x86-bsd-nat.c (x86bsd_dr_set): Use inf->non_exited_threads ().