Check if socket server is MaxScale service

We need to handle the case that a MaxScale service is used as a
server over Unix domain sockets.

server/core/internal/service.hh

@@ -337,6 +337,14 @@ SListener service_find_listener(Service* service,
  */
 bool service_port_is_used(int port);
 
+/**
+ * @brief Check if a MaxScale service listens on a Unix domain socket
+ *
+ * @param path The socket path to check
+ * @return True if a MaxScale service uses the socket
+ */
+bool service_socket_is_used(const std::string& socket_path);
+
 /**
  * @brief Check if the service has a listener with a matching name
  *