--replace not working under Windows

[alainfrisch]

Under Windows, one needs to remove the original file before renaming the temporary one, or one gets Sys_error("File exists"). Before removing the original file, one needs to close the in-channel (which is a good idea anyway), or one gets Sys_error("foo.ml: Permission denied")

We currently use locally:

      let ic = open_in path in
      try
        indent_channel ic args config out;
        close_in ic;
        match out, need_move with
        | Some src, Some dst -> Sys.remove dst; Sys.rename src dst
        | _, _ -> ()
      with e ->
          close_in ic; raise e

but this should be improved to avoid the double-close when an error is raised by Sys.remove or Sys.rename.