8586d9f88e
We were using the enctype from the PA-TGS-REQ's AP-REQ's Ticket to
decide what key from the service's realm's krbtgt principal to use.
This breaks when: a) we're doing cross-realm, b) the service's
realm's krbtgt principal doesn't have keys for the enctype used in
the cross-realm TGT.
The fix is to pick the correct key (strongest or first, per-config)
from the service's realm's krbtgt principal.