Add auth-private data to struct svc_req, use in svc_auth_gss.

New auth-private data members added to struct svc_req.  Used
in svc_auth_gss to pass sequence number.

Nb., window logic not final.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Re-implement KRB5 PAC handling.

Do nothing unless the local GSSAPI provider supports
gss_get_name_attribute.  If so, get all known PAC attributes,
and release when other gss data is cleaned up.

We do nothing with this information, but a server such as
Ganesha can decode or use the data if interested.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>