pru_connect2
error = pru_connect2(so1, so2);
.pru_connect2 = uipc_connect2,
.pru_connect2 = uipc_connect2,
if (so1->so_proto->pr_usrreqs->pru_connect2)
return (*so1->so_proto->pr_usrreqs->pru_connect2)(so1, so2);
int (*pru_connect2)(struct socket *, struct socket *);