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.

Change-Id: I25666b7ff139ae43364ac04869b14997f3ed0d53
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>