MacOS: don't allow krb5 at login when AD plugin authenticates

MacOS: don't allow krb5 at login when AD plugin authenticates

if AD is being used to verify authentication (e.g. via builtin)
don't allow get krb5 at login to succeed. since a helper does this we can't
grey the option, but we can decline to act on it.

Change-Id: I1f2bb891377c3ed9765f9e58141c77ec54d3ae22
Reviewed-on: http://gerrit.openafs.org/3690
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>