viced: Restrict RXAFS_FlushCPS to administrators

viced: Restrict RXAFS_FlushCPS to administrators

RXAFS_FlushCPS currently can be run by anyone, including
unauthenticated users. Forcing CPS calculation can be a relatively
resource-intensive operation, though, if done frequently enough, and
only should need to be done by administrators. Thus, only let
administrators use it.

Change-Id: Iaedd1e72e542b637070930bf1a0a9eba83a9ab64
Reviewed-on: http://gerrit.openafs.org/7572
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>