Use `default-indent-new-line' instead of `indent-new-comment-line'

Use `default-indent-new-line' instead of `indent-new-comment-line'

* lisp/simple.el (default-indent-new-line): Doc string fix.

* lisp/textmodes/refill.el (refill-post-command-function): Make
default-indent-new-line work as indent-new-comment-line.

* lisp/textmodes/refill.el (refill-post-command-function): Bind
`M-C-j' and `M-j' to default-indent-new-line instead of
indent-new-comment-line to allow overriding via
`comment-line-break-function' (bug#12413).