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/output_init.c

@@ -184,6 +184,7 @@ audio_output_init(struct audio_output *ao, const struct config_param *param,
 	}
 
 	ao->plugin = plugin;
+	ao->always_on = config_get_block_bool(param, "always_on", false);
 	ao->enabled = config_get_block_bool(param, "enabled", true);
 	ao->really_enabled = false;
 	ao->open = false;