(comment-indent): Be a little more robust in case

(comment-indent): Be a little more robust in case
comment-indent-function does funny things.
(comment-normalize-vars): Autoload.
(comment-region-internal): Fiddle with the visibility spec.
(comment-valid-prefix): New function.
(comment-indent-new-line): Use it.  Ignore adaptive-fill-prefixes
that would turn comment-text into non-comment-text.