gssd_krb5: arcfour-hmac support

For arcfour-hmac support, the make_checksum function needs a usage
field to correctly calculate the checksum differently for MIC and
WRAP tokens.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>