[MINOR] config: track "no option"/"option" changes

[MINOR] config: track "no option"/"option" changes

Sometimes we would want to implement implicit default options,
but for this we need to be able to disable them, which requires
to keep track of "no option" settings. With this change, an option
explicitly disabled in a defaults section will still be seen as
explicitly disabled. There should be no regression as nothing makes
use of this yet.