restore: detect that someone failed (v2)

Handle SIGCHLD and if someone failed, nr_in_progress is set to -1.
If crtools notices that nr_in_progress is negative, it kills all
tasks.

v2: * Use named constants for task_entries->start in restorer.c
    * Use SA_NOCLDWAIT when setting sigchild handler,
      this makes sigchild handler simpler.

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