Support serving only some user/group ranges.

[dbaarda]

Often you want to only export non-system users and groups over ldap, since client machines will often have their own system users and groups. It would be nice if there was a way to specify uid and gid ranges to export with arguments like these;

--uid=1000-29999 --gid=100,1000-29999

So use a comma-separated list of uids or uid-ranges, where the default is all.