This is default behaviour but you can fill you own socket file descriptor if you want, using the second overload of moustique_listen:
template <typename G, typename H> int moustique_listen(int listen_fd, G closed_connection_handler, H data_handler);
template <typename G, typename H> int moustique_listen(int listen_fd, G closed_connection_handler, H data_handler);
Great project by the way - I've put it on my workbench-TODO for a little hacking some time in the next few days. I've got a standard 'command processing server' project that I've been hacking on over the years, I might try to hack up a moustique_ integration, just for grins.