viced: add opt to allow admin writes on RO servers

viced: add opt to allow admin writes on RO servers

Add the new option -admin-write to allow write requests from superusers
on file servers running in readonly mode (-readonly). This lets sites
run fileservers in readonly mode for normal users, but allows members of
the system:administrators group to modify content.

Reviewed-on: https://gerrit.openafs.org/13707
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit f5f8b9336919debc5c26c429b12a14b65e0b697c)

Change-Id: Ia627b8c99767a875c1e8d1c69dcb45118df36937
Reviewed-on: https://gerrit.openafs.org/14019
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>