This patch prepares code to handle errors. In the near future
we will handle SIGCHLD. If a restore of one task fails, we will
send a signal to other for completing.

For this we should have ability to wait until all task wills be
restored. This patch does it.

v2: Don't wait children.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>