* lisp/files.el: Use lexical-binding.

* lisp/files.el: Use lexical-binding.
(dir-locals-read-from-file): Remove unused `err' variable.
(hack-dir-local-variables--warned-coding): New var.
(hack-dir-local-variables): Use it to avoid repeated warnings.
(make-backup-file-name--default-function): New function.
(make-backup-file-name-function): Use it as default.
(buffer-stale--default-function): New function.
(buffer-stale-function): Use it as default.
(revert-buffer-insert-file-contents--default-function): New function.
(revert-buffer-insert-file-contents-function): Use it as default.
(insert-directory): Avoid add-to-list.
* lisp/autorevert.el (auto-revert-handler): Simplify.
Use buffer-stale--default-function.