[patch][Win32] Fix CenterWindow() function on multiple monitors

[Ken Takata]

Hi,

The CenterWindow() function in gui_w32.c doesn't work properly on multiple
monitors. E.g.:

1. Open a gvim window on the secondary monitor.
2. Input some texts. (e.g. iHello<Esc>)
3. Press Alt+F4 to close the window.
4. A message box (Save changes to "Untitled"?) will be shown on the primary
monitor. It should be shown on the secondary monitor.

Attached patch fixes the problem.
Also attached a patch for fixing some coding style and indentation problems.

Regards,
Ken Takata