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.

etc/NEWS

@@ -1303,6 +1303,12 @@ will return the URL for that bug.
 This allows for rcirc logs to use a custom timestamp format, than the
 chat buffers use by default.
 
+---
+*** New command 'Buffer-menu-toggle-internal', locally bound to 'I'.
+This command toggles the display of internal buffers in Buffer Menu mode;
+that is, buffers not visiting a file and whose names start with a space.
+Previously, such buffers were never shown.
+
 ** Customize
 
 +++

lisp/buff-menu.el

@@ -100,6 +100,10 @@ as it is by default."
 This is set by the prefix argument to `buffer-menu' and related
 commands.")
 
+(defvar-local Buffer-menu-show-internal nil
+  "Non-nil if the current Buffer Menu lists internal buffers.
+Internal buffers are those whose names start with a space.")
+
 (defvar-local Buffer-menu-filter-predicate nil
   "Function to filter out buffers in the buffer list.
 Buffers that don't satisfy the predicate will be skipped.
@@ -140,6 +144,7 @@ then the buffer will be displayed in the buffer list.")
   "V"           #'Buffer-menu-view
   "O"           #'Buffer-menu-view-other-window
   "T"           #'Buffer-menu-toggle-files-only
+  "I"           #'Buffer-menu-toggle-internal
   "M-s a C-s"   #'Buffer-menu-isearch-buffers
   "M-s a C-M-s" #'Buffer-menu-isearch-buffers-regexp
   "M-s a C-o"   #'Buffer-menu-multi-occur
@@ -197,6 +202,10 @@ then the buffer will be displayed in the buffer list.")
      :help "Toggle whether the current buffer-menu displays only file buffers"
      :style toggle
      :selected Buffer-menu-files-only]
+    ["Show Internal Buffers" Buffer-menu-toggle-internal
+     :help "Toggle whether the current buffer-menu displays internal buffers"
+     :style toggle
+     :selected Buffer-menu-show-internal]
     "---"
     ["Refresh" revert-buffer
      :help "Refresh the *Buffer List* buffer contents"]
@@ -317,6 +326,11 @@ ARG, show only buffers that are visiting files."
   (interactive "P")
   (display-buffer (list-buffers-noselect arg)))
 
+(defun Buffer-menu--selection-message ()
+  (message (cond (Buffer-menu-files-only "Showing only file-visiting buffers.")
+                 (Buffer-menu-show-internal "Showing all buffers.")
+	         (t "Showing all non-internal buffers."))))
+
 (defun Buffer-menu-toggle-files-only (arg)
   "Toggle whether the current `buffer-menu' displays only file buffers.
 With a positive ARG, display only file buffers.  With zero or
@@ -325,9 +339,18 @@ negative ARG, display other buffers as well."
   (setq Buffer-menu-files-only
 	(cond ((not arg) (not Buffer-menu-files-only))
 	      ((> (prefix-numeric-value arg) 0) t)))
-  (message (if Buffer-menu-files-only
-	       "Showing only file-visiting buffers."
-	     "Showing all non-internal buffers."))
+  (Buffer-menu--selection-message)
+  (revert-buffer))
+
+(defun Buffer-menu-toggle-internal (arg)
+  "Toggle whether the current `buffer-menu' displays internal buffers.
+With a positive ARG, display non-internal buffers only.  With zero or
+negative ARG, display internal buffers as well."
+  (interactive "P" Buffer-menu-mode)
+  (setq Buffer-menu-show-internal
+	(cond ((not arg) (not Buffer-menu-show-internal))
+	      ((> (prefix-numeric-value arg) 0) t)))
+  (Buffer-menu--selection-message)
   (revert-buffer))
 
 (define-obsolete-function-alias 'Buffer-menu-sort 'tabulated-list-sort
@@ -667,6 +690,7 @@ See more at `Buffer-menu-filter-predicate'."
         (marked-buffers (Buffer-menu-marked-buffers))
         (buffer-menu-buffer (current-buffer))
 	(show-non-file (not Buffer-menu-files-only))
+        (show-internal Buffer-menu-show-internal)
 	(filter-predicate (and (functionp Buffer-menu-filter-predicate)
 			       Buffer-menu-filter-predicate))
 	entries name-width)
@@ -686,7 +710,8 @@ See more at `Buffer-menu-filter-predicate'."
 	       (file buffer-file-name))
 	  (when (and (buffer-live-p buffer)
 		     (or buffer-list
-			 (and (or (not (string= (substring name 0 1) " "))
+			 (and (or show-internal
+                                  (not (string= (substring name 0 1) " "))
                                   file)
 			      (not (eq buffer buffer-menu-buffer))
 			      (or file show-non-file)