Toggle line highlighting in all buffers (Global Hl-Line mode).
With a prefix argument ARG, enable Global Hl-Line mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.
If `global-hl-line-sticky-flag' is non-nil, Global Hl-Line mode
highlights the line about the current buffer's point in all
windows.
Global-Hl-Line mode uses the functions `global-hl-line-unhighlight' and
`global-hl-line-highlight' on `pre-command-hook' and `post-command-hook'.
(fn &optional ARG)
Non-nil if Global-Hl-Line mode is enabled.
See the command `global-hl-line-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-hl-line-mode'.