Kill old buffers that have not been displayed recently.
The relevant variables are `clean-buffer-list-delay-general',
`clean-buffer-list-delay-special', `clean-buffer-list-kill-buffer-names',
`clean-buffer-list-kill-never-buffer-names',
`clean-buffer-list-kill-regexps' and
`clean-buffer-list-kill-never-regexps'.
While processing buffers, this procedure displays messages containing
the current date/time, buffer name, how many seconds ago it was
displayed (can be nil if the buffer was never displayed) and its
lifetime, i.e., its "age" when it will be purged.
(fn)