rpc: handle rotated gss data for Windows interoperability

The data in Kerberos gss tokens can be rotated.  But we were lazy and
rejected any nonzero rotation value.  It wasn't necessary for the
implementations we were testing against at the time.

But it appears that Windows does use a nonzero value here.

So, implement rotation to bring ourselves into compliance with the spec
and to interoperate with Windows.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>