Add a couple of other sensible button defaults in MessageBox()s.

[originally from svn r5305]

windows/window.c

@@ -1862,7 +1862,8 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
 	    if (!cfg.warn_on_close || session_closed ||
 		MessageBox(hwnd,
 			   "Are you sure you want to close this session?",
-			   str, MB_ICONWARNING | MB_OKCANCEL) == IDOK)
+			   str, MB_ICONWARNING | MB_OKCANCEL | MB_DEFBUTTON2)
+		== IDOK)
 		DestroyWindow(hwnd);
 	    sfree(str);
 	}