unix-groups: Silently ignore failures with /usr/bin/groups.

Otherwise fake users which exist in the config only (like 'backups')
get spurious noise, eg:

$ ssh repos info
hello backups, this is git@repos running gitolite3 v3.04-22-g0d371ac on git 1.8.0

/usr/bin/groups: backups: No such user
...
unix-groups: Deal with coreutils 'groups' output format quirks.
Allow users in the @writable group to toggle writability of @all repos.