Support usnetd as backend

[pothos]

Since the raw backend is quite limiting it would be nice to support usnetd as backend.
This allows to still use the kernel network stack without interference.

The IPC works over Unix Domain Sockets:
https://github.com/ANLAB-KAIST/usnetd#adding-support-for-usnetd-to-other-network-stacks
An example on how to use it is here:
https://github.com/ANLAB-KAIST/usnet_sockets#runtime-configuration-of-nic-access

There you can also find how to use macvtap devices. They also have advantages over RAW sockets and can either be used in bridge mode where they get a different IP address or in passthru mode where they take over the NIC and the kernel network stack can't receive/transmit anymore.

Edit: Here are the smoltcp devices: https://github.com/ANLAB-KAIST/usnet_devices