Here are the changes I have

  - Fix some bugs I introduced in zap_thread
  - Improve the check for traced children in sys_wait4
  - Fix parent links when using CLONE_PTRACE

My thanks to OGAWA Hirofumi for pointing out the first bit.

The only other issue I know of is something else Hirofumi pointed out
earlier; there are problems when a tracing process dies unexpectedly.  I'll
come back to that later.