This patch provides the basic framework for RPCSEC_GSS authentication
in the RPC client. The protocol is fully described in RFC-2203.
Sun has supported it in their commercial NFSv3 and v2 implementations
for quite some time, and it has been specified in RFC3010 as being
mandatory for NFSv4.

  - Update the mount_data struct for NFSv2 and v3 in order to allow them
    to pass an RPCSEC_GSS security flavour. Compatibility with existing
    versions of the 'mount' program is ensured by requiring that RPCSEC
    support be enabled using the new flag NFS_MOUNT_SECFLAVOUR.
  - Provide secure authentication, and later data encryption on
    a per-user basis. A later patch will an provide an implementation
    of the Kerberos 5 security mechanism. SPKM and LIPKEY are still
    being planned.
  - Security context negotiation and initialization are all assumed
    to be done in userland. A later patch will provide the actual upcall
    mechanisms to allow for this.