Windows error codes: Move to a separate module

Windows error codes: Move to a separate module

We're going to add many more of these.

This commit is pure code motion, plus the necessary administrivia, as
I have veried with the following runes:

  $ git-diff HEAD~ | grep '^+' |sort >plus
  $ git-diff HEAD~ | grep '^-' | perl -pe 's/^-/+/' |sort >min
  $ diff -ub min plus |less

The output is precisely the expected `mod` and `use` directives.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>