Default to crypt mode for unix clients

Default to crypt mode for unix clients

Though the protection offered by rxkad, even with rxkad-k5 and rxkad-kdf, is
insufficient to protect traffic from a determined attacker, it remains the
case that the internet is not a safe place for user data to travel in the
clear, and has not been for a long time.  The Windows client encrypts by
default, and all or nearly all the Unix client packaging scripts set crypt
mode by default.  Catch up to reality and default to crypt mode in the
Unix cache manager.

Change-Id: If0061ddca3bedf0df1ade8cb61ccb710ec1181d4
Reviewed-on: https://gerrit.openafs.org/12668
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>