Make syslogd work in case shutdown() is POSIX-ly correct.

On POSIX conformant systems, shutdown() should return ENOTCONN when not
connected. We attempted to fix this once (kern/84761), but this change
got backed out because it 'breaks code' (r150155).

I just reapplied the patch and indeed, syslogd fails on startup. Make it
easier to re-enable this change in the future by paching up syslogd to
do the right thing.

MFC after: 3 weeks
Sponsored by: Nuxi