volser: allow clonevol purge id to be new id

volser: allow clonevol purge id to be new id

effectively the same functionality that reclone already uses, but
for some reason we artificially limit it out of clone despite
the interface being there for it. it used to be there. put it back.

Reviewed-on: http://gerrit.openafs.org/6250
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 641c67473615e80cfb8cf1e67636a82e42e5c899)

Change-Id: I31df948a21639bd93c573c77207f0f6c9e43deed
Reviewed-on: http://gerrit.openafs.org/6761
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>