restore: clean up code, which synchronizes resume of tasks (v2)

I added two mechanism of synchronization. The second one is better.
This patch deletes the first one.

Before we had an entry (pid and lock) for each tasks and all this
entries were shared between all processes. Now we don't need "lock"
and we use pids from crtools to kill all processes if someone failed.

v2: s/malloc/xmalloc

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