From David Ponce <david@dponce.com>
(ruler-mode-header-line-format-old): Don't `make-variable-buffer-local'.
(ruler-mode-ruler-function): Default to `ruler-mode-ruler'.
(ruler-mode-header-line-format): Simply funcall the above.
(ruler-mode): Use `make-local-variable' and `kill-local-variable'
to save/restore a previous header line format.
(ruler-mode-space): Don't depend on a numeric WIDTH value.
(ruler-mode-ruler): Use symbolic display elements for scrollbar,
fringes and margins width.
(ruler-mode-ruler-function): Default to ruler-mode-ruler