Unix CM: Generalise token storage

Unix CM: Generalise token storage

This generalises token storage in the Unix CM, so that it isn't
rxkad specific. We add a new, dynamically allocated, list of tokens
hanging off each unixuser structure. Each token is expressed as
a discrimated union keyed on the security class of that token,
with the token's details contained within that Union.

All token handling is performed through a set of functions in
afs_token.c - token access is modified to use this interface
throughout the rest of the code.

Change-Id: I939f3a611bb6e991e1e0d075ced0a59fc6f57693
Reviewed-on: http://gerrit.openafs.org/2580
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>