(comment-search-forward): Make sure we search forward.

(comment-search-forward): Make sure we search forward.
(comment-enter-backward): Try and distinguish the non-matching case at
EOB from the non-matching case with a missing comment-end-skip for
a 2-char comment ender.
(comment-choose-indent): New function extracted from comment-indent.
Improve the alignment algorithm.
(comment-indent): Use it.