* event-loop.h: Mention async_event_handlers.

	* event-loop.h: Mention async_event_handlers.
	(async_event_handler): Forward declare.
	(async_event_handler_func): New typedef.
	(create_async_event_handler, delete_async_event_handler)
	(mark_async_event_handler): Declare.
	* event-loop.c (event_data): New.
	(event_handler_func): Take an event_data instead of an integer.
	(struct gdb_event): Replace the integer file descriptor by a
	generic event_data.
	(async_event_handler): New.
	(async_handler_ready): Delete.
	(async_event_handler_list): New.
	(create_event): New.
	(create_file_event): Use it.
	(process_event): Adjust.
	(gdb_do_one_event): Poll from the event sources in round-robin
	fashion across calls.  Be sure to consult all sources before
	blocking.
	(handle_file_event): Take an event_data instead of an integer.
	Adjust.
	(gdb_wait_for_event): Add `block' argument.  Handle it.
	(mark_async_signal_handler): Remove unneeded cast.
	(invoke_async_signal_handler): Rename to ...
	(invoke_async_signal_handlres): ... this.  Return true if any was
	handled.
	(check_async_ready): Delete
	(create_async_event_handler): New.
	(mark_async_event_handler): New.
	(struct async_event_handler_data): New.
	(invoke_async_event_handler): New.
	(check_async_event_handlers): New.
	(delete_async_event_handler): New.
	(handle_timer_event): Adjust.