This fixes a number of bugs in the thread-release code:

 - notify parents only if the group leader is a zombie,
   and if it's not a detached thread.

 - do not reparent children to zombie tasks.

 - introduce the TASK_DEAD state for tasks, to serialize the task-release
   path. (to some it might be confusing that tasks are zombies first, then
   dead :-)

 - simplify tasklist_lock usage in release_task().

the effect of the above bugs ranged from unkillable hung zombies to kernel
crashes. None of those happens with the patch applied.