s3:winbind: Add support for storing KRB5 credential in KCM

This can store crentiials in the Kerberos Credential Manager e.g.
provided by sssd.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>

Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Mon Jul  1 19:22:02 UTC 2019 on sn-devel-184
