Merge pam into the kauth configure option

Merge pam into the kauth configure option

Realistically, you shouldn't be using either kauth or pam.  The
pam functionality provided by the module in our tree is only
useful in a kaserver-style environment, so it makes sense to merge
the two knobs.

Retain a separate enable_pam variable so that it can be overridden
on a per-architecture basis where it is known to not work.  Consolidate
the two places where we did such checks, as well.

Change-Id: I6bf39ee5002f943548c51d089fe612f7e2f0501b
Reviewed-on: http://gerrit.openafs.org/11524
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>