Fix a narrow-to-region vs. overlays-at bug

Fix a narrow-to-region vs. overlays-at bug

See bug#58703.

* src/buffer.c (overlays_in): Add a new TRAILING arg expressing the
behavior wanted by `overlays-at', namely to include all overlays
beginning at the POS passed to `overlays-at', even if POS is the end
of the narrowed region.  Pass true and the search range is extended to
ZV+1 if END is greater than ZV, just as is done for EMPTY.
(overlays_at): Pass 'true' for the new trailing arg.  At present this
is the only caller passing 'true'.
(mouse_face_overlay_overlaps): Pass 'false' for the new trailing arg.
(disable_line_numbers_overlay_at_eob): ditto.
(Foverlays_in): ditto.
* src/editfns.c (overlays_around): ditto.
* test/src/buffer-tests.el (sorted-overlays): Add a spot test for
this.