Rewrite code quitting windows and restoring the previous buffer.

Rewrite code quitting windows and restoring the previous buffer.

* window.el (set-window-buffer-start-and-point): Call
set-window-start with NOFORCE argument t.  Suggested by Thierry
Volpiatto <thierry.volpiatto@gmail.com>.
(quit-window): Reword doc-string.  Handle new format of
quit-restore parameter.  Don't delete window if it has a
previous buffer we can show instead of the present one.
(display-buffer-record-window): Rewrite using a new format for
the quit-restore window parameter
(special-display-popup-frame, display-buffer-same-window)
(display-buffer-reuse-window, display-buffer-pop-up-frame)
(display-buffer-pop-up-window, display-buffer-use-some-window):
Adapt symbol passed to display-buffer-record-window.
* help.el (help-window-setup): Handle new format of quit-restore
parameter.