(open-line, split-line, next-line): Use `newline'

(open-line, split-line, next-line): Use `newline'
function to insert newline so it will be marked hard if appropriate.
(indent-new-comment-line): Use `newline' function, unless called
with (new) argument.
(do-auto-fill): Call indent-new-comment-line with argument to
make newline soft.