Refactor out sysconfig handling

NP: I would not pass comment as an argument, since ```write_option``` can read it by itself. It does not only add a unneeded argument but also makes things a little big confusing, since the caller would probably expect the comment he is explicitly passing to be always written (even if there is one already there). I know the behaviour is "documented" in a test, but it doesn't fit with the principle of least astonishment (to me, at last).