--idle=oneshot

This is a bit of followup on commit https://github.com/mpv-player/mpv/commit/685b8b7a00ad171f76aba6a93ffaff890aa0caa1

Posting PR to get review since I've never touched this code (although the change is rather trivial). Would it be better to leave `idle` alone and add another option to avoid breaking config files around, or we don't care?