Support mod_auth_kerb
    
    Add support for mod_auth_kerb back, depending on which module
    has been loaded.
    
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Pierre-Yves Chibon <pingou@pingoured.fr>
    Reviewed-by: Howard Johnson <merlin@merlinthp.org>