macos: update AFS prefs pane

macos: update AFS prefs pane

modify afshelper to just run what it's told instead
of offering fixed operations. this avoids having a setuid
tool around. in spite of apple's suggestion this is correct,
it's actually more dangerous. instead, elevate privilege only
to run a small set of commands, then drop. allow
unlocking of the prefs pane, but make the menu extra prompt
for authentication when needed.
deactivate controls in the prefs pane when locked.

Change-Id: I0e2510efc98605f464e04d26cc8f2ce2bfe116f4
Reviewed-on: http://gerrit.openafs.org/7196
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>