This changes add_com to take a cmd_const_cfunc_ftype, and then fixes
up all the command implementations.

In most cases this is trivial.

In a couple of places I had to again introduce a temporary non-const
overload.  These overloads will be removed when add_info is
constified.

gdb/ChangeLog
2017-11-07  Tom Tromey  <tom@tromey.com>

	* solib.h (no_shared_libraries): Constify.
	* frame.h (return_command): Constify.
	* cli/cli-cmds.h (quit_command): Constify.
	* top.h (quit_command, execute_command): Constify.
	* target.h (flash_erase_command): Constify.
	* inferior.h (set_inferior_args, attach_command): Constify.
	* tracepoint.h (start_tracing, stop_tracing): Constify.
	* breakpoint.h (break_command, tbreak_command)
	(hbreak_command_wrapper, thbreak_command_wrapper)
	(rbreak_command_wrapper, watch_command_wrapper)
	(awatch_command_wrapper, rwatch_command_wrapper)
	(get_tracepoint_by_number): Constify.
	* symtab.c (info_variables_command, rbreak_command)
	(symtab_symbol_info): Constify.
	(info_variables_command): Add non-const overload.
	* top.c (dont_repeat_command): Constify.
	* breakpoint.c (ignore_command, commands_command)
	(condition_command, tbreak_command, hbreak_command)
	(thbreak_command, clear_command, break_command)
	(info_breakpoints_command, watch_command, rwatch_command)
	(awatch_command, trace_command, ftrace_command, strace_command)
	(trace_pass_command, break_range_command, dprintf_command)
	(agent_printf_command, get_tracepoint_by_number)
	(watch_maybe_just_location, trace_pass_command): Constify.
	(info_breakpoints_command): Add non-const overload.
	* tracefile.c (tsave_command): Constify.
	* infcmd.c (attach_command, disconnect_command, signal_command)
	(queue_signal_command, stepi_command, nexti_command)
	(finish_command, next_command, step_command, until_command)
	(advance_command, jump_command, continue_command, run_command)
	(start_command, starti_command, interrupt_command)
	(run_command_1, set_inferior_args, step_1): Constify.
	* inferior.c (add_inferior_command, remove_inferior_command)
	(clone_inferior_command): Constify.
	* linux-fork.c (checkpoint_command, restart_command): Constify.
	* windows-nat.c (signal_event_command): Constify.
	* guile/guile.c (guile_repl_command, guile_command): Constify.
	* printcmd.c (x_command, display_command, printf_command)
	(output_command, set_command, call_command, print_command)
	(eval_command): Constify.
	(non_const_set_command): Remove.
	(_initialize_printcmd): Update.
	* source.c (forward_search_command, reverse_search_command):
	Constify.
	* jit.c (jit_reader_load_command, jit_reader_unload_command):
	Constify.
	* infrun.c (handle_command): Constify.
	* memattr.c (mem_command): Constify.
	* stack.c (return_command, up_command, up_silently_command)
	(down_command, down_silently_command, frame_command)
	(backtrace_command, func_command, backtrace_command_1): Constify.
	(backtrace_command): Add non-const overload.
	* remote-sim.c (simulator_command): Constify.
	* exec.c (set_section_command): Constify.
	* tracepoint.c (tdump_command, trace_variable_command)
	(tstatus_command, tstop_command, tstart_command)
	(end_actions_pseudocommand, while_stepping_pseudocommand)
	(collect_pseudocommand, teval_pseudocommand, actions_command)
	(start_tracing, stop_tracing): Constify.
	* value.c (init_if_undefined_command): Constify.
	* tui/tui-stack.c (tui_update_command): Constify.
	* tui/tui-win.c (tui_refresh_all_command)
	(tui_set_tab_width_command, tui_set_win_height_command)
	(tui_set_focus_command, tui_scroll_forward_command)
	(tui_scroll_backward_command, tui_scroll_left_command)
	(tui_scroll_right_command, parse_scrolling_args, tui_set_focus)
	(tui_set_win_height): Constify.
	* tui/tui-layout.c (tui_layout_command): Constify.
	* procfs.c (proc_trace_syscalls, proc_trace_sysentry_cmd)
	(proc_trace_sysexit_cmd, proc_untrace_sysentry_cmd)
	(proc_untrace_sysexit_cmd): Constify.
	* remote.c (threadlist_test_cmd, threadinfo_test_cmd)
	(threadset_test_cmd, threadlist_update_test_cmd)
	(threadalive_test): Constify.
	* objc-lang.c (print_object_command): Constify.
	* command.h (add_com): Constify.
	* cli/cli-dump.c (restore_command): Constify.
	* cli/cli-cmds.c (pwd_command, echo_command, quit_command)
	(help_command, complete_command, shell_command, edit_command)
	(list_command, disassemble_command, make_command)
	(apropos_command, alias_command): Constify.
	* cli/cli-script.c (document_command, define_command)
	(while_command, if_command, validate_comname): Constify.
	* cli/cli-decode.c (struct cmd_list_element): Change type of
	"fun".
	* target.c (do_monitor_command, flash_erase_command): Constify.
	* regcache.c (reg_flush_command): Constify.
	* reverse.c (reverse_step, reverse_next, reverse_stepi)
	(reverse_nexti, reverse_continue, reverse_finish)
	(save_bookmark_command, goto_bookmark_command)
	(exec_reverse_once): Constify.
	* python/python.c (python_interactive_command, python_command):
	Constify.
	* typeprint.c (ptype_command, whatis_command, whatis_exp):
	Constify.
	* solib.c (sharedlibrary_command, no_shared_libraries): Constify.
	* gcore.c (gcore_command): Constify.