... So, I'm almost persuaded to undo the changes in soreceive() which
pass `struct proc *p' to (*pr->pru_usrreq)(), and go back to passing
`(struct proc *)0' where we used to do that.  The struct proc * wasn't
needed before, there's no compelling reason to make it necessary.

That still leaves the dom_externalize/unp_externalize() call, and the
(currently ifdef'ed out) message accounting. Modulo those, a NULL
uio->uio_procp should be allowable and safe.

Does that make a difference?