Change pru_rcvd() return type to the type of void. We have no interest

Change pru_rcvd() return type to the type of void. We have no interest
on pru_rcvd() return value.

Drop "pru_rcvd != NULL" check within pru_rcvd() wrapper. We only call it
if the socket's protocol have PR_WANTRCVD flag set. Such sockets are
route domain, tcp(4) and unix(4) sockets.

ok guenther@ bluhm@