Don't track the per-device configuration in NMDnsManager by
the ifname, but by the ifindex. We should consistently treat
the ifindex as the ID of a link, like kernel does.

At the few places where we actually need the ifname, resolve
it by looking into the platform cache. That is not necessarily
the same as the ifname that is currently tracked by NMDevice,
because netdev interfaces can be renamed, and NMDevice updates
it's link properties delayed. However, the platform cache has
the most recent notion of the correct interface name for an
ifindex, so if we ever hit a race here, we do it now more
correctly.

This also temporarily drops support for mdns. Will be re-added next,
but differently.