SUNRPC: Add support for privacy to generic gss-api code.

 Add support for privacy to generic gss-api code.  This is dead code until we
 have both a mechanism that supports privacy and code in the client or server
 that uses it.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>