kdc: add enable_fast option (enabled by default)

This makes it possible to disable fast if needed,
which is very good for regression tests.
Samba disables this in a few test environments with this
option.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15002
BUG: https://bugzilla.samba.org/show_bug.cgi?id=15005

Signed-off-by: Stefan Metzmacher <metze@samba.org>

kdc/krb5tgs.c

@@ -902,6 +902,9 @@ validate_fast_ad(astgs_request_t r, krb5_authdata *auth_data)
 
     krb5_data_zero(&data);
 
+    if (!r->config->enable_fast)
+	return 0;
+
     ret = _krb5_get_ad(r->context, auth_data, NULL,
 		       KRB5_AUTHDATA_FX_FAST_USED, &data);
     if (ret == 0) {