rxgk: Define some protocol constants

rxgk: Define some protocol constants

rxgk_int.xg is missing a few constants mentioned in the respective
protocol specs:

- The RPC-L definitions for PrAuthName are defined, but no
  PRAUTHTYPE_* constants for the 'kind' field are defined. Define at
  least PRAUTHTYPE_GSS, which rxgk uses.

- The rxgk spec indicates a size of 20 for the nonces used in rxgk
  challenge and response packets. Define a constant
  (RXGK_CHALLENGE_NONCE_LEN) for this value, to make it easier to
  define similarly-sized structures.

- The rxgk-afs spec defines the time value of 0 as a special "never
  expires" value. Define a constant (RXGK_NEVERDATE) to represent it.

Change-Id: I07e1a1b19d1c887fd3e1a1d0f270d5af7b8581b0
Reviewed-on: https://gerrit.openafs.org/12939
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>