Members of the thread group must die without reporting to the parent and
without going to zombie stage. We do that by reparenting to init before
catching a SIGKILL. The parent will not see the child death.
The thread group leader must report the exit status, even if it exits
because of another thread calling exit_group(). We do that by storing the
exit status in struct linux_emuldata_shared, and the exit hook has the
duty of setting struct proc's p_xstat for the thread group leader.
2) For exit/fork/exec hooks, move the NPTL specific code to separate functions
that are shared between COMPAT_LINUX and COMPAT_LINUX32
3) Fix LINUX_CLONE_PARENT_SETTID semantics
81c909dd45