When looking for "group daemon" (since that's what's in mtree), make sure

When looking for "group daemon" (since that's what's in mtree), make sure
we actually look for the *group* and not the user's gid.  user daemon
has traditionally been group 31 (guest).

Also clear out the groups vector so that it doesn't inherit the groups
of the invoking user (ever run rwhod by hand before?)  Unfortunately, we
can't empty the supplemental groups list because the !&@^#! egid is stored
in there! :-(