Ticket 49165 pw_verify did not handle external auth
    
    Bug Description:  During the change to improve sasl and simple bind,
    we externalised the backend selection outside of do_bind. In an
    auto_bind scenario however, this mean the be was null, causing the
    dn to always be invalidated.
    
    Fix Description:  Add a pw_validate_be_dn function, that correctly
    checks if we are anonymous, a real be dn, or rootdn. This then allows
    the correct authentication of autobinds.
    
    https://pagure.io/389-ds-base/issue/49165
    
    Author: wibrown
    
    Review by: mreynolds (Thanks!)