Disable kauth by default

We should actively be discouraging the use of the kaserver and related
utilities.

The src/kauth/ directory will still be compiled, just not installed.
(If we stopped compiling it, it would likely bitrot very quickly so
as to become unbuildable, and having it still build seems a reasonable
goal given our obligations with respect to compatibility with IBM
AFS for the use of the AFS name.)

Change-Id: Ib710af3e177223d85cd9c6099ce75e700b6a2958
Reviewed-on: http://gerrit.openafs.org/10962
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>