idmap: allow ranges with external mapping to overlap
    
    If POSIX IDs are managed externally e.g. by AD it might be possible that
    the IDs are centrally manages for the whole forest. Hence there might
    not be a single ID range for each member domain in the forest but only a
    single ID range for the whole forest. This means that we have to allow
    collisions if ID ranges in this case.
    
    Unit tests are added to make sure that the collisions are only allowed
    for external mappings.