(comment-styles): New `box-multi'.
(comment-normalize-vars): Better default for comment-continue to
avoid whitespace-only continuations.
(comment-search-forward): Always move even in the no-syntax case.
(comment-padright): Only obey N if it's only obeyed for padleft.
(comment-make-extra-lines): Better handling of empty continuations.
Use `=' for the filler if comment-start has only one character.
(uncomment-region): Try handling the special `=' filler.
(comment-region): Allow LINES even if MULTI is nil.
(comment-box): Choose box style based on comment-style.