Fix nested group memberships
    
    Search the local db to find the local DN using the original DN as search key.
    This way we do not have to rely on weak and faulty heuristicts based on DN
    names.
    Add a few helper functions in the process and change the way we pass members to
    sysdb_store_group_send(), instead of passing users and groups list, just add
    member DNs to the other sysdb attrs.