output_plugin: added methods enable() and disable()

With these methods, an output plugin can allocate some global
resources only if it is actually enabled.  The method enable() is
called after daemonization, which allows for more sophisticated
resource allocation during that method.

src/output/pulse_output_plugin.c

@@ -309,6 +309,8 @@ pulse_output_init(G_GNUC_UNUSED const struct audio_format *audio_format,
 
 	pa_threaded_mainloop_unlock(po->mainloop);
 
+	po->stream = NULL;
+
 	return po;
 }