All interpreter types (CLI/TUI/MI) print the prompt, and then call
start_event_loop.

Because we'll need an interpreter hook to display the
interpreter-specific prompt before going back to the event loop,
without actually starting an event loop, this patch moves the
start_event_loop call to common code, and replaces the command_loop
hook with a pre_command_look hook, that now just prints the prompt.

Turns out to be a cleanup on its own right anyway.

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* cli/cli-interp.c (cli_interpreter_pre_command_loop): New
	function.
	(cli_interp_procs): Install it instead of cli_command_loop.
	* cli/cli-interp.h (cli_interpreter_pre_command_loop): Declare.
	* event-top.c (cli_command_loop): Delete.
	* interps.c (interp_new): Remove reference to command_loop_proc.
	(current_interp_command_loop): Delete.
	(interp_pre_command_loop): New function.
	(interp_command_loop_ftype): Delete.
	* interps.h (interp_pre_command_loop_ftype): New typedef.
	(struct interp_procs) <command_loop_proc>: Delele field.
	<pre_command_loop_proc>: New field.
	(current_interp_command_loop): Delete declaration.
	(interp_pre_command_loop): New declaration.
	* main.c (captured_command_loop): Call interp_pre_command_loop
	instead of current_interp_command_loop and start an event loop.
	* mi/mi-interp.c (mi_command_loop): Delete.
	(mi_interpreter_pre_command_loop): New.
	(mi_interp_procs): Update.
	* tui/tui-interp.c (tui_interp_procs): Install
	cli_interpreter_pre_command_loop instead of cli_command_loop.