restore: sync threads

Use the save mechanosm as for processes.

* Threads should starts together with processes
* If a thread segfaulted, we should be able to detect it.

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