volinfo: remove code duplication in HandleVolumes

Reduce duplicate code for processing volume header files when printing header
details (-header) and calculating the volume size (-sizeOnly).

Reviewed-on: http://gerrit.openafs.org/4695
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>

(cherry picked from commit ae27283550dab33704f30e18975722e0ed2c5424)

Change-Id: I83e5797acf09597d7c4da8c90e49049cec1294db
Reviewed-on: http://gerrit.openafs.org/11248
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>