gss_krb5: split up functions in preparation of adding new enctypes

Add encryption type to the krb5 context structure and use it to switch
to the correct functions depending on the encryption type.

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>