Toggle Linum mode in all buffers.
With prefix ARG, enable Global-Linum mode if ARG is positive;
otherwise, disable it. If called from Lisp, enable the mode if
ARG is omitted or nil.
Linum mode is enabled in all buffers where
`linum-on' would do it.
See `linum-mode' for more information on Linum mode.
(fn &optional ARG)
Non-nil if Global-Linum mode is enabled.
See the command `global-linum-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 `global-linum-mode'.