restorer: implement restore of ps tree in stopped state

This patch adds implementation of --leave-stopped option
to CRIU restore cmd. If --leave-stopped is passed then
each process in ps tree gets SIGSTOP before detaching
from it.

Signed-off-by: Kravchenko Dmitrii <equivalence1@gmail.com>
Signed-off-by: Eugene Batalov <eabatalov89@gmail.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>