s4-kdc: restore MIT KDC backend

s4-kdc: restore MIT KDC backend

Fix fallout from the KDC prefork patchset (99aea42520fc..).

GCC warns when Samba is being built with --with-system-mitkrb5.
Fix this by adapting the signature of mitkdc_task_init() to match
task_init which has been extended to return a status code.

Status codes try to mimick those of kdc-heimdal.c:kdc_task_init()
as closely as possible.

Signed-off-by: Philipp Gesang <philipp.gesang@intra2net.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>