auth: Add more tests and resulting fixes to userok

auth: Add more tests and resulting fixes to userok

Add tests for the functions afsconf_SuperUser() and
afsconf_SuperIdentity(). These had been missing tests because testing
them requires starting a client and a server, so amend the superuser-t
tests so that they can start up a simple server.

Fix a number of problems that the tests expose, with setting (and
freeing) identities in corner cases.

Change-Id: I29f5f9eda7f532c98183d588e488d704f8efad88
Reviewed-on: http://gerrit.openafs.org/3593
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>