bind() to AF_UNIX will now require unveil "w". "w" may seem a little odd

bind() to AF_UNIX will now require unveil "w".  "w" may seem a little odd
(and it may seem it should be "r" to get access to the file to collect
the underlying socket, which is fully r/w in a non-file way).  But this
matches the POSIX spec that the file be 'writeable'.  The regress test
and daemons have been updated for this behaviour.
Gap discovered by martijn, long discussions with benno