Abstract /vicepX header traversal

Abstract /vicepX header traversal

The code for examining all of the headers on a vice partition now exists
in two places: the VGC scanner, and the salvager. Create the
VWalkVolumeHeaders function that contains the common logic so it's in
one place, and take a couple of function callbacks for the differences.

Change-Id: I12c71b3344ffbd0b22ddf5418f9ff0509734f42b
Reviewed-on: http://gerrit.openafs.org/1264
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>