Shutdown sockets before closing

The server socket does not release the port it was listening for if we
just close it: we must also shutdown it.

app/src/server.c

@@ -71,6 +71,7 @@ static socket_t listen_on_port(Uint16 port) {
 
 static void close_socket(socket_t *socket) {
     SDL_assert(*socket != INVALID_SOCKET);
+    net_shutdown(*socket, SHUT_RDWR);
     if (!net_close(*socket)) {
         LOGW("Cannot close socket");
         return;