Don't use file notification on non-file buffers

Don't use file notification on non-file buffers

Allow non-file buffers to declare that notification on their
default-directory is sufficient to know when auto-revert updates are
required by setting the new variable `buffer-auto-revert-by-notification'
to non-nil.  If nil, the default, then auto-revert will poll
those buffers instead. (bug#35418).
Currently, only Dired sets that variable.

* lisp/autorevert.el (auto-revert-buffers):
Modify condition for using notification.
* lisp/files.el (buffer-auto-revert-by-notification): New variable.
* lisp/dired.el (dired-mode): Set buffer-auto-revert-by-notification.
* doc/emacs/arevert-xtra.texi (Non-File Buffers): Document new variable.
* etc/NEWS (Changes in Specialized Modes and Packages): Describe new variable.