whitespace: Redo BoB/EoB empty line highlighting

* lisp/whitespace.el (whitespace--empty-at-bob-matcher,
whitespace--empty-at-eob-matcher, whitespace--update-bob-eob,
whitespace-color-off, whitespace-color-on,
whitespace-empty-at-bob-regexp, whitespace-empty-at-eob-regexp,
whitespace-looking-back, whitespace-post-command-hook): Redo the
`empty' line highlighting logic to ensure that a buffer change causes
all affected `empty' lines to become (un)highlighted (bug#37467).
Also, for improved UX, don't highlight BoB empty lines at or below
point (not just when point is at 1), or EoB empty lines at or above
point (not just when point is `eobp').
(whitespace-bob-marker, whitespace-eob-marker): Clarify documentation.
* test/lisp/whitespace-tests.el (whitespace--with-test-buffer,
whitespace--fu, whitespace-tests--empty-bob,
whitespace-tests--empty-eob): Add tests.

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