This ensures that IPv6-only systems get working mirrors.
configs/releng/airootfs/etc/xdg/reflector/reflector.conf: Reduce the amount mirrors that reflector checks from 70 to 20. This significantly reduces the time it takes to end up with an up-to-date mirrorlist during boot with the releng profile. Fixes #92
archiso specific options are placed in a /etc/systemd/system/reflector.service.d/archiso.conf drop-in.