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(hexl-mode--old-var-vals): New var to replace them.
(hexl-mode--minor-mode-p, hexl-mode--setq-local): New funs.
(hexl-mode, hexl-follow-line, hexl-activate-ruler):
Use them to set local vars.
(hexl-mode-exit): Use hexl-mode--old-var-vals to restore state.
(hexl-backward-short, hexl-backward-word, hexl-scroll-down)
(hexl-scroll-up, hexl-end-of-1k-page, hexl-end-of-512b-page): Simplify.

Fixes: debbugs:7846