Avoid polling in global-auto-revert-mode (bug#35418)

Avoid polling in global-auto-revert-mode (bug#35418)

Make `auto-revert-avoid-polling' have effect in global-auto-revert-mode.
Buffers actually handled by that mode are marked with a non-nil value
of `auto-revert--global-mode'.  When global-auto-revert-mode is
entered, eligible buffers are marked in that way, and hooks are set up
to mark new buffers and take care of buffers whose file names change.
This way the existing poll-avoidance logic can be used, since the
entire set of buffers in auto-revert is known.

A new hook, `after-set-visited-file-name-hook', was added to handle
the case when the file name of a tracked buffer changes.

* lisp/autorevert.el (auto-revert-avoid-polling): Amend doc string.
(auto-revert--global-mode): New buffer-local variable.
(global-auto-revert-mode): Mark existing buffers and set up hooks when
mode is entered; do the opposite when exited.
(auto-revert--global-add-current-buffer)
(auto-revert--global-adopt-current-buffer)
(auto-revert--set-visited-file-name-advice): New functions.
(auto-revert--polled-buffers, auto-revert--need-polling-p)
(auto-revert-notify-handler)
(auto-revert-active-p): Modify logic to cover global-auto-revert-mode.
* lisp/files.el (after-set-visited-file-name-hook): New hook.
(set-visited-file-name-hook): Call new hook.
* test/lisp/autorevert-tests.el (top):  Use lexical-binding.
(auto-revert-test--write-file, auto-revert-test--buffer-string)
(auto-revert-test--wait-for, auto-revert-test--wait-for-buffer-text)
(auto-revert-test05-global-notify): New test.
* doc/lispref/hooks.texi (Standard Hooks):
Mention new hook (in a comment, since it's unclear whether it should
actually be documented here)
* etc/NEWS (Changes in Specialized Modes and Packages): Update entry.