MXS-1689 Properly check for duplicate port/socket of service

It is now impossible to create two listeners for a service that
would listen on the same port/socket (as before), but the error
message is now sensible and provides detailed information to the
user.

include/maxscale/service.h

@@ -236,6 +236,24 @@ bool serviceAddBackend(SERVICE *service, SERVER *server);
  */
 bool serviceHasBackend(SERVICE *service, SERVER *server);
 
+/**
+ * @brief Find listener with specified properties.
+ *
+ * @param service Service to check
+ * @param socket  Listener socket path
+ * @param address Listener address
+ * @param port    Listener port number
+ *
+ * @note Either socket should be NULL and port non-zero or socket
+ *       non-NULL and port zero.
+ *
+ * @return True if service has the listener
+ */
+SERV_LISTENER* service_find_listener(SERVICE* service,
+                                     const char* socket,
+                                     const char* address,
+                                     unsigned short port);
+
 /**
  * @brief Check if a service has a listener
  *