vlclient: add -probe option

Add a new option to the vlclient test program to call the
probe server RPC to ping the vlservers in a cell. Uses a multi
rx call to do the probes in parallel.

The existing -host option can be used to ping a single
vlserver.

Reviewed-on: http://gerrit.openafs.org/8911
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit bb733da134ce7d7742d5b7359edb36f67ec85632)

Change-Id: I9d26e072fafa3e785bb1bc2eb0d8db43fbc1ff57
Reviewed-on: http://gerrit.openafs.org/9570
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>