volinfo: implement -checkout option

Implement the -checkout option using the FSSYNC communication
channel with the fileserver.

Reviewed-on: http://gerrit.openafs.org/5101
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 752daf71cd75aa707fabb1b4598dd8ef1f51f973)

Change-Id: Ic6614d0a28b968e261c5916e607ac8fd9d00f4d1
Reviewed-on: http://gerrit.openafs.org/11266
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>