apply weak key exceptions to _kdc_get_preferred_key
apply weak key exceptions to _kdc_get_preferred_key

As part of the keytype validity checks within _kdc_get_preferred_key
_kdc_is_weak_exception must be used to permit the afs/* principals
to have only DES in the key list.

Change-Id: I70801ce9b8c4d3f057542541ce11e06d195efd52

madscientist159/heimdal,madscientist159/heimdal,madscientist159/heimdal,madscientist159/heimdal,madscientist159/heimdal,madscientist159/heimdal,madscientist159/heimdal