[krbdev.mit.edu #7646] PAC checksum verification failed with enterprise principals

[Greg Hudson via RT]

I guess we'd want to parse with KRB5_PRINCIPAL_PARSE_ENTERPRISE | 
KRB5_PRINCIPAL_PARSE_IGNORE_REALM.

The presence of an enterprise principal in the ticket is just as 
confusing to me as its presence in the PAC.  But if these are being 
issued by a real AD server, I won't argue with experimental evidence.