Add pac_user_get_grp_info() to read current group memberships
To be able to efficiently store group memberships we need to know the
current memberships of a user. sysdb_initgroups() is used to read the
user entry together with all groups the user is a member of. Some of the
group attributes are kept to avoid additional lookups and speed up
further processing.
Currently sysdb_initgroups() does not return the original DN of the
group. Since it is needed to remove memberships later on it is added to
the list of requested attributes