This will allow specifying more options per address than the
simple Address= entry in the [Network] section.
Preliminary support for the same functionality for [Route] sections
are added, but not yet hooked up, as more testing is needed.
This removed the requirement for devices to be tagged with
'systemd-networkd' before they will be visible to networkd.
Still, as by default we don't ship any .network files, network
devices will simply be tracked, but not touched, unless the
admin configures things explicitly.
