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/decoder_thread.c

@@ -167,8 +167,6 @@ stop_no_close:
 
 static void * decoder_task(mpd_unused void *arg)
 {
-	notify_enter(&dc.notify);
-
 	while (1) {
 		assert(dc.state == DECODE_STATE_STOP);