MacOS: aklog auth plugin

MacOS: aklog auth plugin

the AuthorizationPlugin subsystem replaced the old loginplugin
method. As it is now usable for us, provide a plugin which can take
advantage of it.

LICENSE BSD

Reviewed-on: http://gerrit.openafs.org/3939
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit c3499218ad604b1985239334d77dd0a94c7128bb)

Change-Id: I8280d0f1c2ac6ffb93ddc42a5cc5234401a4fcbc
Reviewed-on: http://gerrit.openafs.org/4090
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>