Fix handling of old `display-buffer' options (bug#8851) (bug#8856).

Fix handling of old `display-buffer' options (bug#8851) (bug#8856).

* window.el (display-buffer-normalize-specifiers-1): Don't
check pop-up-frames for 'unset initialization.
(display-buffer-normalize-specifiers-2): Major rewrite using
special-display-p and same-window-p (Bug#8851) and (Bug#8856).
(pop-up-frames, display-buffer-reuse-frames)
(display-buffer-mark-dedicated): Don't initialize to 'unset.
Suggested by David Engster <deng@randomsample.de>.
(even-window-heights): Initialize to 'unset.
(display-buffer-alist-set): Handle new 'unset initializations.