New functions to switch back and forth to another major mode

New functions to switch back and forth to another major mode

* subr.el (major-mode--suspended): New var.
(major-mode-suspend, major-mode-restore): New funs, extracted from doc-view.
* doc-view.el (doc-view--previous-major-mode): Remove.
(doc-view-mode): Use major-mode-suspend.
(doc-view-fallback-mode): Use major-mode-restore.
* hexl-mode.el (hexl-mode--minor-mode-p, hexl-mode--setq-local): Remove.
(hexl-mode): Use major-mode-suspend and hexl-follow-ascii-mode.
(hexl-mode-exit): Use major-mode-restore.
(hexl-activate-ruler, hexl-follow-line): Don't bother trying to preserve
earlier state, now that entering/leaving hexl-mode kills local vars.
(hexl-follow-ascii-mode): New proper local minor mode.
(hexl-follow-ascii): Rewrite, using it.
* image-mode.el (image-mode-previous-major-mode): Remove.
(image-mode): Use major-mode-suspend.
(image-mode-to-text): Use major-mode-restore.