usnet_devices: Ethernet interface types for smoltcp, usnet_sockets, and usnetd

See usnet_sockets and usnetd for a description.

This here is kept as separate repository and not moved to smoltcp nor to usnetd or usnet_sockets for the following reasons. The TAP device type is modified from smoltcp, and if the correct MTU would be used in smoltcp, just the macvtap code could be moved in smoltcp if accepted there. But anyway smoltcp does not need to have netmap support and even zero-copy forwarding in its repository and it makes more sense to maintain this separately.

Warning for Netmap at the current state

Passing the result from AsRawFd into some syscall like poll is unsafe even if there is a library that gives you a wrapper hiding unsafe. This is due to how the head pointer is currently set which netmap uses to swap the buffers with implicit TX on poll. To allow any other thread to run poll at any time this library here would need to forward the head pointer only when the buffer was fully forwarded. (Details are not in my head anymore.)

Features

The netmap feature is optional and requires the netmap and netmap_user C headers to be available for compilation. At runtime the netmap kernel module must be loaded if netmap is to be used.