output_thread: reimplement CANCEL synchronization

The output thread could hang indefinitely after finishing CANCEL,
because it could have missed the signal while the output was not
unlocked in ao_command_finished().

This patch removes the wait() call after CANCEL, and adds the flag
"allow_play" instead.  While this flag is set, playback is skipped.
With this flag, there will not be any excess wait() call after the
pipe has been cleared.

This patch fixes a bug that causes mpd to discontinue playback after
seeking, due to the race condition described above.

src/output_init.c

@@ -189,6 +189,7 @@ audio_output_init(struct audio_output *ao, const struct config_param *param,
 	ao->really_enabled = false;
 	ao->open = false;
 	ao->pause = false;
+	ao->allow_play = true;
 	ao->fail_timer = NULL;
 
 	pcm_buffer_init(&ao->cross_fade_buffer);