[S21.5] simple.el GNU syncs, next-line/previous-line

I'm also not terribly happy about the sync of `next-line' and `previous-line'. Those syntax errors are a pretty reliable indicator of buggy code in my experience. I've run into these errors several times, and each time converting to `forward-line' was simple, and either fixed bugs or made correctness obvious where it was not with `next-line'. Non-nil `next-line-add-newlines' was always a potential problem, and when moving to the goal column was useful, you need careful analysis to be sure the `next-line' behavior is correct for you. Please check whether converting vc to use `forward-line' is better idea.