volser: Convert command line parsing to cmd

Convert argument parsing to use the cmd library.

Change-Id: I2bf9d7ce482d49f50cec59a1660a431bae17017a
Reviewed-on: http://gerrit.openafs.org/9827
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>