kdc: map KRB5_PROG_SUMTYPE_NOSUPP to KRB5KDC_ERR_SUMTYPE_NOSUPP

RFC4120 says KRB5KDC_ERR_SUMTYPE_NOSUPP should be returned if the KDC does not
support a given checksum type. Return this instead of KRB5_PROG_SUMTYPE_NOSUPP
by introducing a new wrapper function, _kdc_verify_checksum().

kdc/digest.c

@@ -586,9 +586,9 @@ _kdc_do_digest(krb5_context context,
 	if (ret)
 	    goto out;
 
-	ret = krb5_verify_checksum(context, crypto,
+	ret = _kdc_verify_checksum(context, crypto,
 				   KRB5_KU_DIGEST_OPAQUE,
-				   buf.data, buf.length, &res);
+				   &buf, &res);
 	free_Checksum(&res);
 	krb5_data_free(&buf);
 	krb5_crypto_destroy(context, crypto);