Add code to make autoswap work on Windows.

Some changes to attempt to make this work on Windows using +clientserver.  Not perfect, but works.
Hi, thank you for the PR!

Maybe it is possible to simplify all this code using something like 

```vim
let fname_esc = substitute(a:filename, "'", "''", "g")
let servername = ...
if remote_expr(servername, "bufloaded('" . fname_esc . "')")
    return servername
endif
```

Have a look at <https://github.com/svintus/vim-editexisting/blob/master/plugin/editexisting.vim#L22-L71>.

This technique is not applicable for vim inside a terminal and for some gvim instances, but it is possible that it works fine for win32.