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.

Change-Id: I53a2a503812237a850170c39c81ee3fb56c8282e
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>
doc: volscan man page

Provide a man page for the volscan utility

Change-Id: Ibaecb2b9030ee71d81f13b897694c4cf3b4b9516
Reviewed-on: http://gerrit.openafs.org/10905
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>