kerberos: make sure we only use prompter type when available.

kerberos: make sure we only use prompter type when available.

We also verified that we cannot simply remove the prompter as several older
versions of Heimdal would crash.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11038

Guenther

Signed-off-by: Günther Deschner <gd@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Günther Deschner <gd@samba.org>
Autobuild-Date(master): Fri Oct  2 07:29:43 CEST 2015 on sn-devel-104

(cherry picked from commit 6755376cedaf0c88230b47e04c584c7d9fce13e3)

Autobuild-User(v4-3-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-3-test): Mon Oct  5 12:44:42 CEST 2015 on sn-devel-104