that all involved principals has a des key in the database, and that
the client has a version 4 (un-)salted key. Furthermore
krb5_425_conv_principal has to do it's job, as present it's not very
clever.
git-svn-id: svn://svn.h5l.se/heimdal/trunk/heimdal@2642 ec53bebd-3082-4978-b11e-865c3cabbd6b