Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Drop all received data mbufs from a socket's queue if the MT_SONAME
mbuf is dropped, to preserve the invariant in the PR_ADDR case.

Add a regression test to detect this condition, but do not hook it
up to the build for now.

PR: kern/38495
Submitted by: James Juran
Reviewed by: sam, rwatson
Obtained from: NetBSD
MFC after: 2 weeks