vos: Unrecognized or ambiguous switch '-c'

The documented abbreviation '-c' for '-cell' no longer works since
the -config option was added.  Add a parm alias '-c' to restore
compatibility with previous syntax.

Change-Id: Ie98e470d204ea2186a95668bba2581cf3e88f729
Reviewed-on: http://gerrit.openafs.org/9276
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>