Toggle Msb mode.
With a prefix argument ARG, enable Msb mode if ARG is positive,
and disable it otherwise. If called from Lisp, enable the mode
if ARG is omitted or nil.
This mode overrides the binding(s) of `mouse-buffer-menu' to provide a
different buffer menu using the function `msb'.
(fn &optional ARG)
Non-nil if Msb mode is enabled.
See the command `msb-mode' for a description of this minor mode.
Setting this variable directly does not take effect;
either customize it (see the info node `Easy Customization')
or call the function `msb-mode'.