Make 'n'/'p' in image mode buffers respect dired sorting

Make 'n'/'p' in image mode buffers respect dired sorting

The commands now also now work on archive and tar mode parent buffers.

* doc/emacs/files.texi (Image Mode): Document it.

* lisp/arc-mode.el (archive-goto-file): New function (bug#38647).
(archive-next-file-displayer): Ditto.

* lisp/image-mode.el (image-next-file): Reimplement to work on
displayed dired buffers and the like.  This means that `n' and `p'
now works on the displayed ordering in the dired buffer, so if
you've reversed the sorting, `n' picks the right "next" file.
(image-mode--directory-buffers): New function.
(image-mode--next-file): Ditto.

* lisp/tar-mode.el (tar-goto-file): New function.
(tar-next-file-displayer): Ditto.