Properly handle MSKRB5 by passing sec=mskrb5 to the upcall so that the
spengo blob can be generated appropriately. Also, make
decode_negTokenInit prefer whichever mechanism is first in the list.

Needed for some NetApp servers, and possibly some older
versions of Windows which treat the two KRB5 mechanisms differently.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>