restore: Bring original SIGCHLD handler back once task is restored

Otherwise if restored task exit we get error message in form

 | Error (cr-restore.c:1302): 7244 exited, status=0

while restored task simply finished working.

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