Split default-buffer-default into separate display action components.

Split default-buffer-default into separate display action components.

* lisp/window.el (display-buffer-alist): Doc fix.
(display-buffer): Add docstring.  Don't treat display-buffer-default specially.
(display-buffer-reuse-selected-window)
(display-buffer-same-window, display-buffer-maybe-same-window)
(display-buffer-reuse-window, display-buffer-pop-up-frame)
(display-buffer-pop-up-window)
(display-buffer-reuse-or-pop-window)
(display-buffer-use-some-window): New functions.
(display-buffer-default-action): Use them.
(display-buffer-default): Deleted.
(pop-to-buffer-1): Fix choice of actions.