Implement advisory? file locks on Windows

[plajjan]

For feature parity... now that ReadFile/WriteFile has locks, we need something similar on Windows.

It's really just advisory locking we want / need I think, since that's all we can get on POSIX anyway, I think!? but that's enough for implementation of lock files and similar.

I've used file locking in haskell which has some support for Windows, so copy with pride!