Whine if single-DES keys are in use

If we are using single-DES keys in our KeyFile, yell at the
administrator, so they have a chance at realizing that they should
migrate to stronger crypto.

Change-Id: Ic37d9e1cea7ee7e12594be0dec02000f11efc896
Reviewed-on: http://gerrit.openafs.org/10273
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>