Bug#4356: menu-bar-mode flag argument is inconsistent with universe

[eichin@kitten.gen.ma.us (Mark W. Eichin)]

> In 19.31 this has changed so that `nil' doesn't have the desired
> effect.  Instead, you have to supply a negative number !

Hmm. The way elisp "optional arguments" work, you can't distinguish
between (menu-bar-mode nil) and (menu-bar-mode), and both are
equivalent to "M-x menu-bar-mode" which a user might actually type.
scroll-bar-mode works the same way -- no arg (or nil) means *toggle*,
positive arg means turn on, negative arg means turn off.
(auto-compression-mode is another example)

In earlier releases, I vaguely recall that (menu-bar-mode) from the
user's .emacs file worked, but I don't have an older version of emacs
handy for comparison or to identify the change.