Prefix userdata hives with _ to avoid conflicts
    
    The main userdata dict contains common attributes, but we add
    a sepcial groups list and unmapped extras, as well as indicators
    like auth_type.
    All these additional attributes are now prefixed by a _ character
    so that conflicts with legitimate attributes are improbable.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>