Eliminate immediate_quit

Eliminate immediate_quit

This finally gets rid of immediate_quit (and surrounding
infrustruture), as nothing sets it anymore.

gdb_call_async_signal_handler was only necessary in order to handle
immediate_quit.  We can just call mark_async_signal_handler directly
on all hosts now.

In turn, we can clean up mingw-hdep.c's gdb_select a bit, as
sigint_event / sigint_handler is no longer needed.

gdb/ChangeLog:
2016-04-12  Pedro Alves  <palves@redhat.com>

	* defs.h: Update comments on SIGINT handling.
	(immediate_quit): Delete declaration.
	* event-loop.c (call_async_signal_handler): Delete.
	* event-loop.h (call_async_signal_handler): Delete declaration.
	(mark_async_signal_handler): Update comments.
	(gdb_call_async_signal_handler): Delete declaration.
	* event-top.c (handle_sigint): Call mark_async_signal_handler
	instead of gdb_call_async_signal_handler.
	* exceptions.c (prepare_to_throw_exception): Remove reference to
	immediate_quit.
	(exception_fprintf): Remove comments about immediate_quit.
	* mingw-hdep.c (sigint_event, sigint_handler): Delete.
	(gdb_select): Don't wait on sigint_event.
	(gdb_call_async_signal_handler): Delete.
	(_initialize_mingw_hdep): Delete.
	* posix-hdep.c (gdb_call_async_signal_handler): Delete.
	* utils.c (immediate_quit): Delete.