invariants and approach for protocol switch methods in protsw_init(),
and also some KASSERT's for non-domain init entries in protocol
switch tables: pru_abort and pru_send must both be implemented.
For now, leave those assertions #if 0'd, since there are a few
protocols that violate them in non-harmful ways. Whether or not we
should enforce pru_abort being implemented for non-stream protocols
is an interesting question: currently abort is only invoked on stream
sockets in situations where un-accepted sockets must be abruptly
closed (i.e., close() on a listen socket with pending connections),
but in principle it is useful for datagram sockets and most datagram
socket types implement it.
MFC after: 3 weeks
9c232f86ca