crtools: restore nested mount namespaces (v2)

Known issue:
* currently only namespaces with the same root is supported
* nested namespaces can be dumped and restored only if the root task
  has own mount namespace.

All nested namespaces are restored in a root namespace in temporary
directories. All mount points restored in one tree and then they are
divided into namesaces.
The task with minimal pid for each namespaces unshared mntns and
then it makes pivot_root in a proper temporary directory. All other
tasks makes setns to enter into a mount namespace of the task with
minimal pid.

v2: clean up

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>