auth: Let superuser identities be superusers

auth: Let superuser identities be superusers

We have a special rx_identity_kind for superusers, let it actually
be useful for something.

Reviewed-on: https://gerrit.openafs.org/10575
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit fe8a1f3a2b669057451cac358faa7320722dc053)

Change-Id: I36b6b6be6851c45b13c41e751e4d741cd0130fdb
Reviewed-on: https://gerrit.openafs.org/13103
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>