(split-height-threshold, split-width-threshold): Add

(split-height-threshold, split-width-threshold): Add
choice nil.
(split-window-preferred-function): Allow either nil or a function.
(window--splittable-p, window--try-to-split-window): Handle
changed option values.
(window--frame-usable-p): Handle nil argument.
(display-buffer): Call get-lru-window when pop-up-windows is nil
and window can't be split.