protection. Since this requires checksumming an entire request,
instead of just the header, and since the request may include,
for example, pages with write data, we modify the gss_api
routines to pass xdr_bufs instead of xdr_netobjs where
necessary.

We add rpcauth_wrap_req and rpcauth_unwrap_resp to rpcauth.c,
wrappers for the new rpc cred ops crwrap_req and crunwrap_req,
which are called just before encoding, and just after decoding,
respectively.