[krbdev.mit.edu #8925] [Comment] qualify_shortname default can be harmful in LAN setups

https://krbdev.mit.edu/rt/Ticket/Display.html?id=8925
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* s4u2proxy (k5_get_proxy_cred_from_kdc()) does not do fallback on the service
name. (Neither does krb5_tkt_creds_step() or IAKERB, but that was known at the
time of fallback implementation.)