Maybe already conclusion is met, so probably I shouldn't post this patch
but... here is the patch implementing msg80934. I believe this is thread
safe. (This patch doesn't include codes of fopen.patch, but it doesn't
mean they are not necessary. My patch is just a minimal patch for CRT hook)

I'm not sure which way is the best, but I'm happy if popup dialog on
test_os.py (test_fdopen) goes away. I'm startled every time. ;-)