This restores the original behavior of 'overlays_in'. Changes in
this behavior had been made for cases of narrowing, but this
resulted in a regression with uses of 'remove-overlays'.
* src/buffer.c (overlays_in): Change all occurrences of ZV to Z.
* test/src/buffer-tests.el (test-overlays-in-2)
(test-remove-overlays): Adjust expected results to accommodate
changes in 'overlays_in'.