(fortran-mode): Set comment-padding to "$$$".

(fortran-mode): Set comment-padding to "$$$".
Add fortran-comment-line-start-skip to comment-start-skip.
(fortran-comment-indent): Keep whole-line comments in column 0.
(fortran-find-comment-start-skip): New arg `all'.
If ALL is nil, make sure we only match comment-start-skip if we
can't match fortran-comment-line-start-skip.
Fix bug that made it return t but without moving point when
matching '!'! (a false-comment followed by a real comment).
(fortran-indent-comment): Use new `all' argument above.
Be careful not to add an incorrect comment-starter like "C"
in comment-column.
(fortran-split-line): When splitting a comment, reuse the comment
starter from the current line rather than fortran-comment-line-start.
(fortran-indent-line, fortran-auto-fill): Simplify thanks to the
cleaner behavior of fortran-find-comment-start-skip.
(fortran-fill): Don't be confused by ! inside a comment.
(fortran-break-line): Minor cleanup and simplification.