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.

Reviewed-on: http://gerrit.openafs.org/7196
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 0524e0d1915f519a76d62540b695abc16634d4c9)

Change-Id: Iba4435829a57e91e45b0beea10a8d6573b62a214
Reviewed-on: http://gerrit.openafs.org/7495
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>