'strace' shows a problem with a missing release_task for self-reaping
clones that have been traced.  We need to defer releasing them until the
tracer is done with them, but if the tracer dies, we need to handle that
case gracefully too. 

We do that by having 'forget_original_parent()' generate a list of tasks
to release when this case happens.

Patch based on discussions on linux-kernel, and suggestions from Roland
McGrath <roland@redhat.com>.