Hi Oleg.

Attaching the fixed patch.
Changes:
- in wait_task_consider(), pr_detach hides the process
only in !ptrace case
- reparenting within the group is no longer re-enables detaching
- in do_notify_parent_cldstop(), do not allow notifications of
real_parent (still allow for ptrace parent)
- scared by your question about reparenting, I reserved the
flags argument for the future extensions. :) Reparenting may
depend on a flag.

The attached patch implements the PR_DETACH prctl
command. It detaches the entire process group from
its parent, allowing the parent to still read the detach
code with normal wait().
Can be used to daemonize process with threads.