pru_accept
error = pru_accept(so, nam);
.pru_accept = uipc_accept,
.pru_accept = tcp_accept,
.pru_accept = tcp_accept,
if (so->so_proto->pr_usrreqs->pru_accept)
return (*so->so_proto->pr_usrreqs->pru_accept)(so, nam);
int (*pru_accept)(struct socket *, struct mbuf *);