[krbdev.mit.edu #2166] gss serializer code missing some fields

[Ken Raeburn via RT]

Looks like these fields need handling: proto (num), cksumtype (num),
acceptor_subkey (keyblock or null, should be consistent with
have_acceptor_subkey), acceptor_subkey_cksumtype (num),
have_acceptor_subkey (num).