This moves the group mapping API into the passdb backend.

This moves the group mapping API into the passdb backend.

Currently this calls back to mapping.c, but we have the framework
to get the information into LDAP and the passdb.tdb (should we? I
think so..).

This has received moderate testing with net rpc vampire and
usrmgr. I found the add_groupmem segfault in add_aliasmem as
well, but that will be another checkin.

Volker