cacheout: Perform authenticated RXAFS_FlushCPS

Fileservers may now require RXAFS_FlushCPS calls to be made with
administrator tokens. So, try to make the call with admin tokens, and
provide the usual -noauth and -localauth options.

Reviewed-on: http://gerrit.openafs.org/7573
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit cb64dea75a114f6ebcba174be4e71c7fb5f85658)

Change-Id: I0d8e38d3cc1b0e55f63cfa0e50fd9cd4cebc8c30
Reviewed-on: http://gerrit.openafs.org/9486
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>