klog.krb5: enforce DES for rxkad

klog.krb5: enforce DES for rxkad

0. Always request a TGT regardless of the state of
   writeTicketFile.

1. request des-cbc-crc when requesting a ticket for an
   rxkad service principal

2. check the returned key length to ensure that it matches
   the permitted length of an rxkad key.  If not, generate
   an error instead of overwriting memory and continuing.

FIXES 130278

Change-Id: Ib7d27acc60c1b38dab4e2a56f83643477afb9721
Reviewed-on: http://gerrit.openafs.org/5619
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>