More Posix stuff on Windows

[Eli Zaretskii]

I'd like to add emulations of a couple of Posix functions to make a
few Guile procedures available on Windows.  Specifically, getaffinity,
setaffinity, getpriority, and setpriority can work on Windows, once
the emulations of the corresponding C APIs are written, which isn't
hard.

Is it OK to add those to win32-proc.c?  (Perhaps the file should be
renamed to posix-w32.c or some such now, since it is no longer only
about processes.)

If you agree, I will post the patches here.