Fix to properly set AP_OPTS_SUBKEY in heimdal. In MIT it is a #define, but

Fix to properly set AP_OPTS_SUBKEY in heimdal.  In MIT it is a #define, but
in heimdal it is an enum.  Thanks to Guenther Deschner (gd@suse.de).

With this join will work, but without a keytab, cifs connections will still
fail with heimdal.  Fix to come later.
(This used to be commit d30bef4c37e8203c273eb3852215a89348bece7a)