notify: protect notify->pending with the mutex

There was a known deadlocking bug in the notify library: when the
other thread set notify->pending after the according check in
notify_wait(), the latter thread was deadlocked.  Resolve this by
synchronizing all accesses to notify->pending with the notify object's
mutex.  Since notify_signal_sync() was never used, we can remove it.
As a consequence, we don't need notify_enter() and notify_leave()
anymore; eliminate them, too.

src/output_thread.c

@@ -67,8 +67,6 @@ static void *audio_output_task(void *arg)
 {
 	struct audio_output *ao = arg;
 
-	notify_enter(&ao->notify);
-
 	while (1) {
 		switch (ao->command) {
 		case AO_COMMAND_NONE: