cmd: exit status zero from -help

cmd: exit status zero from -help

Return of an exit status of zero when running commands with -help,
instead of returning an error. By general convention, and in
previous versions, tools do not not treat -help as an error.

The AFS::Command perl modules, use -help as an introspection
technique, and fail when commands run with the -help option returns
non-zero.

Change-Id: I6ecd95f6ccd07218a2657dbb4dbf1c13599159f4
Reviewed-on: http://gerrit.openafs.org/8087
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>