Optionally show internal buffers in Buffer Menu mode

Optionally show internal buffers in Buffer Menu mode

Internal buffers were never shown before but they can be of interest
to Elisp developers, especially since there is no general mechanism to
remove unused buffers.

* lisp/buff-menu.el (Buffer-menu-show-internal)
(Buffer-menu--selection-message, Buffer-menu-toggle-internal): New.
(Buffer-menu-mode-map): Bind to `I`.
(Buffer-menu-mode-menu): Add menu entry.
(list-buffers--refresh): Extend filtering logic.
* etc/NEWS: Announce.