This fixes the lockup.

The bug happened because reparenting in the CLONE_THREAD case was done in
a fundamentally non-atomic way, which was asking for various races to
happen: eg. the target parent gets reparented to the currently exiting
thread ...

(the non-CLONE_THREAD case is safe because nothing reparents init.)

the solution is to make all of reparenting atomic (including the
forget_original_parent() bit) - this is possible with some reorganization
done in signal.c and exit.c. This also made some of the loops simpler.