event_pipe: added pipe_event enum and callbacks

Make the event_pipe (formerly main_notify) send/receive a set of
events, with a callback for each one.

The default event PIPE_EVENT_SIGNAL does not have a callback.  It
is still there for waking up the main thread, when it is waiting for
the player thread.

src/database.c

@@ -59,7 +59,6 @@ db_init(void)
 
 	do {
 		event_pipe_wait();
-		reap_update_task();
 	} while (isUpdatingDB());
 
 	stats.numberOfSongs = countSongsIn(NULL);