Do not loop on principal unknown errors

Do not loop on principal unknown errors

If the canonicalize flag is set, the MIT KDC always return the client
principal when KRB5_KDC_ERR_C_PRICIPAL_UNKNOWN is returned.

Check that this is really a referral by testing that the returned
client realm differs from the requested one.

[ghudson@mit.edu: simplified and narrowed is_referral() contract.
Note that a WRONG_REALM response with e-data or FAST error padata
could now be passed through k5_preauth_tryagain() if it has an empty
crealm or a crealm equal to the requested client realm.  Such a
response is unexpected in practice and there is nothing dangerous
about handling it this way.]

(cherry picked from commit d5755694b620570defeecee772def90a2733c6cc)

ticket: 8060
version_fixed: 1.13.1