crtools: Namespaces support skeleton

New option -n to dump/restore namespaces.

Fork the namespaces dumping task and write a helper for switching a namespace.

Prepare the restorer code for restoring namespaces before root task.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>