Add stopping and starting of listeners to maxadmin

Listeners can now be stopped and started individually.

include/maxscale/service.h

@@ -225,6 +225,15 @@ bool serviceLaunchListener(SERVICE *service, SERV_LISTENER *port);
  */
 bool serviceStopListener(SERVICE *service, const char *name);
 
+/**
+ * @brief Restart a stopped listener
+ *
+ * @param service Service where the listener is linked
+ * @param name Name of the listener
+ * @return True if listener was restarted
+ */
+bool serviceStartListener(SERVICE *service, const char *name);
+
 /**
  * Utility functions
  */