kdc: add referral_policy callback to windc plugin

Add a referral policy hook to the TGS as a more elegant way of resolving
referral detection for Samba). The hook can either rewrite the server_princ in
the request, or it can return an error to disable built-in referral processing.

tests/plugin/windc.c

@@ -123,6 +123,7 @@ static krb5plugin_windc_ftable windc = {
     pac_generate,
     pac_verify,
     client_access,
+    NULL, /* referral_policy */
     finalize_reply
 };