Additional fixes in handling of buffer display specifiers.

Additional fixes in handling of buffer display specifiers.

* window.el (display-buffer-default-specifiers): Remove
pop-up-frame.  Add pop-up-window-min-height,
pop-up-window-min-width, and another reuse-window specifier
(Bug#8882).  Reported by Dan Nicolaescu <dann@gnu.org>.
(display-buffer-normalize-specifiers-2): Handle
split-height-threshold and split-width-threshold also when
pop-up-windows is unset.  Add a reuse-window specifier for the
case popping up a new window fails.
(special-display-popup-frame): Remove double quoting.