Prefix userdata hives with _ to avoid conflicts

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>