WIN-28: fix POSIX file descriptor numbering assumptions

[Tom Tromey]

>>>>> "Bryce" == Bryce McKinlay <bryce@waitaki.otago.ac.nz> writes:

Bryce> I recall seeing a patch from Jeff a long time ago to fix
Bryce> FileDescriptor to be more portable and not make the POSIX
Bryce> file-descriptor-is-an-int assumption, but it never got checked
Bryce> in.

As I recall it used a RawData to represent the file descriptor.
I think we should resurrect that.

Tom