This makes UdpRepr work like IpRepr, where it only emits the header, and the user
must emit the payload.
This makes it easier to emit UDP packets with payloads that come from protocol-specific
reprs, like DHCP and in the future DNS.
DeviceCapabilities contains the `medium` field, so tests had to give it a value
even if it was unused. This is impossible to do with no `medium-*` enabled, because
it makes `Medium` uninhabited (empty enum).
- Add `medium` in `DeviceCapabilities`.
- Rename EthernetInterface to Interface.
- Add support to Interface for both Ethernet and IP mediums. The medium to use is detected from `device.capabilities().medium`.
- Ethernet-only features are gated behind the "ethernet" feature, as before.
- IP features are always enabled for now.
qiujiangkun
Dario Nieuwenhuis
Ryan Summers