This routine enable or disable a Menu for you, without you having to have the
Menu's address hard coded into your program.
Give it the Menu list and the number that identifies the intended Menu, and
it will find that Menu and, depending on the Boolean 'On', it will set or
clear the MENUENABLED bit in the Menu's Flags.
If you are dealing with a Menu that is already installed (with SetMenuStrip),
you can call: