Introduce 3 states we will have to work with:

* alive for tasks sleeping or running
* dead for zombies
* stopped for stopped tasks. We cannot distinguish tasks in this state now,
  but with freezer cgroup this will become possible
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>