pru_listen
error = pru_listen(so);
.pru_listen = uipc_listen,
.pru_listen = uipc_listen,
.pru_listen = tcp_listen,
.pru_listen = tcp_listen,
if (so->so_proto->pr_usrreqs->pru_listen)
return (*so->so_proto->pr_usrreqs->pru_listen)(so);
int (*pru_listen)(struct socket *);