bug#22000: Patch addressing the menu-bar frame-resize interaction

[Vivek Dasmohapatra]

On Mon, 15 Oct 2018, martin rudalics wrote:

> Finally, I would provide a 'menu-bar-scrollbar-mode' that would add
> the 'menu-bar-scrollbar' parameter automatically.  Please have a look
> at how for example 'window-divider-mode' (which is off by default)

I have a command which cycles through the available scrollbar modes
and sets the "next" value in default-frame-alist & applies it to all
extant frames: However I'm aware that -mode means something specific
in elisp and this is not that. It's not a toggle as most minor modes
are but a 4-state (or 3-state for 3.16-).

Any suggestions on a name? Or is -mode still acceptable?
(currently called `menu-bar-scrollbar-mode' since I had
 to call it _something_).