MFC r268601:

Make bind(2) and connect(2) return EAFNOSUPPORT for AF_UNIX on wrong
address family.

See https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=191586 for the
original discussion.

Reviewed by:	terry
MFC r268643:

Document that listen(2) can fail with EDESTADDRREQ.
MFC r268787:

Deprecate m_act.  Use m_nextpkt always.
MFC r268825:

Remove deprecated definition "m_act".

Suggested by:	adrian, glebius