s3-client: Add a KRB5 wrapper for smbspool

s3-client: Add a KRB5 wrapper for smbspool

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11690

We need a wrapper for smbspool to be able to authenticate with Kerberos.
This needs to replace the cups smb backend. The permission need to be
0700 and the owner root.

Note that Kerberos support is broken in CUPS 2.1.2 maybe earlier
versions. It works with 1.6.3.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Michael Adam <obnox@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Mon Jan 25 19:58:46 CET 2016 on sn-devel-144