Disallow creating users with ANONYMOUSID

It can result only in sadness.

Document this restriction alongside UID 0 as a reserved number.

Change-Id: Ibea2d98bc15a730bc85e84477791ca45a40f2d92
Reviewed-on: http://gerrit.openafs.org/10950
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>