comment-filling fixes

[Dave Love]

These changes fix previously-reported problems with filling comments
that got ignored as far as I know.

1. M-q on the second line would fill all three comments on to the
   first line.

  ...                  ; ...
  ;; ...
  ;; ...

2. M-q on a line in a Lisp doc string containing a semicolon could
   mess up the filling, e.g.

(defun foo ()
  "foo bar.
foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo; foo foo
bar"

went to

(defun foo ()
  "foo bar.
foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo; foo
                                                               ; foo
bar"

newcomment.el.diff
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fill.el.diff
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