restore: mount proc for a new pid namespace

Create a tmp directory and mount proc from a target pid ns.
This proc will show pid-s from the target pid ns.

crtools uses map_files for restoring sharing mappings.

the tmp directory is removed after restore.

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