Use normal codepath for win32

[hhugo]

The PR removes the "degraded" code path when Sys.win32 is true.

It goes in the direction of #330 but leaves a bunch of diffs out for now.

• It doesn't normalize path '/' vs \
• It doesn't search for a shell (bash.exe, dash.exe, sh.exe) and unconditionally uses bash.
• It doesn't protect against command line length limit
• It doesn't contain the following "fix" (not sure what the bug is ...)

let prep_windows_cmd cmd =
  (* workaround known ocaml bug, remove later *)
  if String.contains cmd '\t' && String.contains cmd ' ' = false then
    " " ^ cmd
  else
    cmd

[hhugo]

One should probably wait for #336 before merging so that we can see that it works without any hacks from the sunset repo.

[hhugo]

I've merged #330, this is now ready for review. cc @kit-ty-kate @gasche

[kit-ty-kate]

apart from the couple of comments below that looks fine to me