Add image-auto-resize defcustoms to image-mode.el

Add image-auto-resize defcustoms to image-mode.el

* lisp/image-mode.el (image-auto-resize)
(image-auto-resize-on-window-resize): New defcustoms.
(image-mode-map): Bind "sb" to image-transform-fit-both.
(image-mode): Set image-transform-resize to image-auto-resize initially.
(image-mode--setup-mode): Add hook on image-auto-resize-on-window-resize.
(image-toggle-display-image): Check if image-transform-resize is t.
(image-transform-properties): Check image-transform-resize for nil and t.
(image-transform-fit-both): New command.
(image-transform-reset): Reset image-transform-resize to image-auto-resize.

* doc/emacs/files.texi (Image Mode): Mention image-auto-resize and
image-auto-resize-on-window-resize.

https://lists.gnu.org/archive/html/emacs-devel/2020-04/msg01160.html