* lisp/midnight.el: Make it a minor mode. Allow predicates.

* lisp/midnight.el: Make it a minor mode.  Allow predicates.

* lisp/midnight.el: Use lexical-binding.
(midnight-mode): Make it a proper minor mode.
(midnight-buffer-display-time): Make arg non-optional.
(midnight-find): Remove.
(clean-buffer-list-kill-never-regexps)
(clean-buffer-list-kill-regexps): Tweak type for new function choice.
(clean-buffer-list-delay): Allow clean-buffer-list-kill-regexps to
contain functions.
(clean-buffer-list): Use cl-find.
Allow clean-buffer-list-kill-never-regexps to contain functions.