volinfo: refactor vnode handling

At program startup, register procedures to be called
when scanning vnodes.

Reviewed-on: http://gerrit.openafs.org/5100
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 11f54962f638c8ae48d5193f6b695c252fbb9937)

Change-Id: Iafee77acb6b175c6ba3983889c8c4adad17aaf79
Reviewed-on: http://gerrit.openafs.org/11264
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>