WIN-28: fix POSIX file descriptor numbering assumptions

[Andrew Haley]

Boehm, Hans writes:
 > > 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 .

Indeed.  Mind you, running 32-bit anything on Itanium would be very
silly.  And it's going to break a *lot* of code...

Andrew.