Hi - have been busy for a while but finally put this together.

The attached patch series:

  - Lets the user control menubar scrolling via a frame parameter.
  - Handles initial-frame-alist
  - Handles default-frame-alist
  - Handles the modify-frame-parameter path (this is secretly how initial
    frames work).
  - Documents the new frame parameter.

Patches prepared on top of the emacs-26 branch but should apply to master
too.

Let me know if the series needs any revision (I have already done the 
copyright-assignment dance).