fix RT#88840, don't terminate a child fork psuedo process in DLL Loader Lock

fix RT#88840, don't terminate a child fork psuedo process in DLL Loader Lock

TerminateThread will terminate a thread but leaks all resources
of that thread, and all locks will never be released, as documented in MSDN.
There is no alternative to locks not being released that I see, but atleast
-e "if ($pid=fork){kill(9,$pid)} else {sleep 5}"
in fork.t won't deadlock with this patch since win32_start_child be reached before
TerminateThread happens. The 5 ms timeout can be increased if problems
arise in the future. The HWND of the child is delivered by win32_start_child
very early, before any perl bytecode is executed, therefore the delay is
keeping in spirit with a kill 9. In any case, if the child thread fails
to schedule, (a DllMain in DLL_THREAD_ATTACH of some DLL in the process
deadlocks or does very long (over 5 ms right now) sync IO), the parent interp
will bail out.