Make Python use a struct serial event

Make Python use a struct serial event

Now that we have an abstract for wakeable events, use it instead of a
(heavier) serial pipe.

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

	* python/python.c: Include "ser-event.h".
	(gdbpy_event_fds): Delete.
	(gdbpy_serial_event): New.
	(gdbpy_run_events): Change prototype.  Use serial_event_clear
	instead of serial_readchar.
	(gdbpy_post_event): Use serial_event_set instead of serial_write.
	(gdbpy_initialize_events): Use make_serial_event instead of
	serial_pipe.