Make `display-buffer' emulate obsolete Emacs 23 options.

Make `display-buffer' emulate obsolete Emacs 23 options.

* window.el (window-sides-alist): Remove.
(display-buffer): Remove defgroup and all its members.
(display-buffer-normalize-specifiers-1): Mention argument in
doc-string.
(display-buffer-normalize-specifiers-2): New function.
(display-buffer-normalize-specifiers): Call
display-buffer-normalize-specifiers-2.
(display-buffer-alist-set): Take care of unset initial values.
Add entry for display-buffer-mark-dedicated.  Reorder.
(same-window-buffer-names, same-window-regexps)
(special-display-frame-alist, special-display-function)
(special-display-buffer-names, special-display-regexps)
(pop-up-frame-alist, pop-up-frame-function)
(split-window-preferred-function, split-height-threshold)
(split-width-threshold, even-window-heights): Restore Emacs 23
values, doc-strings and customization groups.
(pop-up-frames, pop-up-windows, display-buffer-reuse-frames)
(display-buffer-mark-dedicated): Restore Emacs 23 doc-strings and
customization groups.  Initialize value to 'unset.
(display-buffer-reuse-window): Quote lambda with #'.