pulse-server: make some modules load-once

Add mechanism for allowing some modules to be loaded only once.

Mark always-sink, raop-discover, switch-on-connect, zeroconf-discover as
such.

src/modules/module-protocol-pulse/module.h

@@ -37,6 +37,7 @@ struct pw_properties;
 
 struct module_info {
 	const char *name;
+	unsigned int load_once:1;
 	struct module *(*create) (struct impl *impl, const char *args);
 };