Revive display-buffer-function, add a few more buffer display functions

Revive display-buffer-function, add a few more buffer display functions
and adjust callers.

* window.el (display-buffer-same-frame-specifiers)
(display-buffer-other-window-same-frame-specifiers): New
constants.
(display-buffer): Call display-buffer-function if it's non-nil.
(display-buffer-same-window, display-buffer-other-window)
(pop-to-buffer-other-window, switch-to-buffer-other-window):
Adjust doc-strings.
(display-buffer-same-frame)
(display-buffer-other-window-same-frame)
(pop-to-buffer-same-frame)
(pop-to-buffer-other-window-same-frame)
(switch-to-buffer-same-frame)
(switch-to-buffer-other-window-same-frame): New functions.
(display-buffer-function): Un-obsolete.

* bookmark.el (bookmark-bmenu-2-window): Use
pop-to-buffer-other-window.
(bookmark-bmenu-switch-other-window): Use
display-buffer-other-window.

* progmodes/etags.el (find-tag-other-window): New argument
other-frame. Call either pop-to-buffer-other-window or
pop-to-buffer-other-frame.
(find-tag-other-frame): Call find-tag-other-window with
other-frame argument t.

* speedbar.el (speedbar-find-file-in-frame)
(speedbar-buffer-click): Use pop-to-buffer-other-frame.

* term/ns-win.el (ns-find-file): Use pop-to-buffer-other-frame.

* erc-speedbar.el (erc-speedbar-goto-buffer): Use
pop-to-buffer-other-frame.

* windows.texi (Displaying Buffers): Document
display-buffer-function revival and the -same-frame,
-other-window-same-frame postfixed functions.