volinfo: separate volscan binary

Refactor vol-info.c into several files and change the makefile to
build a separate volscan binary, instead of using the program name
to determine if the user is running volinfo or volscan.

This commit adds new source files for the volinfo and volscan main()
function and a common header file.

Reviewed-on: http://gerrit.openafs.org/10903
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit af2e3d81cff39ed06e3bfbfcbfff52163c503c54)

Change-Id: Ie261ac5a8bd6baea7d5d5110248611219ee1c175
Reviewed-on: http://gerrit.openafs.org/11278
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>