cacheout: Add -encrypt option

Add the -encrypt option to cacheout, for forcing communication with
servers over encrypted channels.

Change-Id: I1a2a9faf2fb72238102b018b7692a6691a7c0054
Reviewed-on: http://gerrit.openafs.org/9941
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>