Resync rxgk_int.xg with draft-wilkinson-afs3-rxgk-afs-05

Mostly shuffling around the order of things, but also
adds a cb_id field to the token contents.

Change-Id: Ie07b91d1dd64b6945e096c9aab133015a101f5ab
PRFplus: return the output length as well

Change-Id: Ie3000cb5bcdc6528257586b423f6daf92cdfea70
Shoehorn new token format support

Change-Id: I3b722f9732168566c1f30b0fa62eac35e14aabcb
Move some token-handling helpers to rxgk_token.c

Change-Id: Ie3eb48a59c22dd91b0f5a843bf402b88ddd39777