output: added option "always_on" for radio stations

Did you ever accidently click "stop" while feeding a radio station?
This option sets the output device to "pause" to disable the "close"
method.  It falls back to "pause" then, which is specific to the
plugin.  Some plugins implement it by feeding silence.

src/player_thread.c

@@ -978,7 +978,7 @@ static gpointer player_task(G_GNUC_UNUSED gpointer arg)
 		case PLAYER_COMMAND_CLOSE_AUDIO:
 			player_unlock();
 
-			audio_output_all_close();
+			audio_output_all_release();
 
 			player_lock();
 			player_command_finished_locked();