afs: maintain afs_users buckets in sorted order

Modify afs_GetUser() to insert a new unixuser into an afs_users
hash bucket in sorted order, by uid/PAG.  This is in support of
other small optimizations in future commits.

Change-Id: I68c51ab38733575290aa0c8aa2a306168c5483c0
Reviewed-on: http://gerrit.openafs.org/11071
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>