user_ldap: really resolve nested groups

The previous patch fixed the problem only for one level of indirection
because groupsMatchFilter() had been applied on each recursive call (and
thus there would be no second level if the first level fails the check).

This new implementation replaces the recursive call with a stack that
iterates all nested groups before filtering with groupsMatchFilter().

Signed-off-by: Roland Tapken <roland@bitarbeiter.net>