output: one thread per audio output

To keep I/O nastiness and latencies away from the core, move the audio
output code to a separate thread, one per output.  The thread is
created on demand, and currently runs until mpd exits.

src/output_init.c

@@ -98,6 +98,10 @@ int audio_output_init(struct audio_output *ao, ConfigParam * param)
 	} else
 		audio_format_clear(&ao->reqAudioFormat);
 
+	ao->thread = 0;
+	notify_init(&ao->notify);
+	ao->command = AO_COMMAND_NONE;
+
 	if (plugin->init(ao, param) != 0)
 		return 0;