WIN-28: fix POSIX file descriptor numbering assumptions

[Boehm, Hans]

> From: Adam Megacz [mailto:gcj@lists.megacz.com]
>
> Gak, it is in fact a pointer. However, I doubt there will ever be such
> a thing as Win64 -- Microsoft is most likely going to use the CLR as
> their 64-bit ABI. Still, just plain Wrong for me to make casts like
> that.
> 
AFAIK, win64 is already here.  See
http://www.microsoft.com/windowsxp/64bit/overview.asp .  Or did I miss
something?

Hans