MXS-1951: Fix port conflict detection

With the addition of SO_REUSEPORT support, it is no longer possible to
rely on the network stack to prevent multiple listeners from listening on
the same port. Without explicitly checking for the ports it would be
possible for two listeners from two different services to listen on the
same port in which case the service would be almost randomly chosen.

server/core/config.cc

@@ -4087,7 +4087,7 @@ int create_new_listener(CONFIG_CONTEXT* obj)
             address = "";
         }
 
-        if (auto l = service_find_listener(service, socket, address, port))
+        if (auto l = listener_find_by_config(socket, address, port))
         {
             string socket_type = socket_defined ? "socket" : "port";
             string socket_definition = socket_defined ? socket : obj->m_parameters.get_string(CN_PORT);