libafs: rename volume accessTime to setupTime

libafs: rename volume accessTime to setupTime

Since OpenAFS 1.0, the struct volume accessTime member has been the time
time the volume structure is setup, not the last time the volume was
used (as indicated by the comments).  This time stamp is only used to
find the oldest available volume slot in the disked backed volume cache.
(Perhaps in pre-OpenAFS this was updated each time the volume was
referenced.)

Rename this structure member and update the comments for it.

Change-Id: I33a6371e8800b2d0f7b2700db0785fc365a8649e
Reviewed-on: https://gerrit.openafs.org/11896
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>