gssd_krb5: More arcfour-hmac support

For the arcfour-hmac support, the make_seq_num and get_seq_num
functions need access to the kerberos context structure.
This will be used in a later patch.

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>