* src/buffer.c (Foverlays_at): Add argument `sorted'.

* src/buffer.c (Foverlays_at): Add argument `sorted'.
* doc/lispref/display.texi (Overlay Properties): Reword the doc of `priority'.
(Finding Overlays): Document new arg of `overlays-at'.
* lisp/obsolete/lucid.el (extent-at):
* lisp/htmlfontify.el (hfy-overlay-props-at): Use the new `sorted' arg of
overlays-at.
(hfy-fontify-buffer): Remove unused var `orig-ovls'.
* lisp/ps-def.el (ps-generate-postscript-with-faces1): Use the new `sorted'
arg of overlays-at.  Use `invisible-p'.