Fix KDC client referrals

Fix KDC client referrals

Although our built-in KDB modules do not support client referrals for
AS requests, the KDC is supposed to return one if a third-party module
returns a DB entry containing a principal in a foreign realm.
Unfortunately, this code has never worked; in prepare_error_as(), we
erroneously compare the protocol code errcode against the com_err code
KRB5KDC_ERR_WRONG_REALM; as a result, we never supply the canonical
client principal.  Fix this by comparing errcode against the protocol
code KDC_ERR_WRONG_REALM instead.

Discovered by Alexander Bokovoy and Simo Sorce.

ticket: 8241 (new)
target_version: 1.13.3
tags: pullup