[krbdev.mit.edu #8718] krb5_get_credentials incorrectly matches user to user ticket

[Greg Hudson via RT]

I am inclined towards option 1, because a user-to-user credential is 
not useful if you are looking for a regular ticket.

However, it seems that we also tag constrained delegation (S4U2Proxy) 
results with the is_skey flag, because kdcrep2creds() just checks 
whether there was a second ticket in the request to set that flag.  
So if we always apply the is_skey field match, we break caching of 
S4U2Proxy results, causing a test failure (t_s4u.py runs t_s4u, which 
fails in check_ticket_count()).

I think setting the is_skey field for S4U2Proxy results is a bug, 
since the is_skey field is documented as "true if the ticket is 
encrypted in another ticket's skey", and tickets resulting from 
S4U2Proxy are encrypted in the service's long-term key.  So I will 
look into fixing that bug first.