vfs: Permit unix sockets to be opened with O_PATH

[markj]

Summary

This is compatible with Linux. In principle connectat(2) and bindat(2)
could be modified to support an AT_EMPTY_PATH-like mode which operates
on the socket referenced by an O_PATH fd referencing a unix socket.
That would eliminate the path length limit imposed by sockaddr_un.
(That is not quite enough to make syslog(3) work in capability mode,
since syslog(3) has to handle the possibility that syslogd was
restarted, in which case we must re-resolve the /var/run/log path.)