1. simplifies and regularizies creation of listen sockets based on the listen
interfaces setting.
2. simplifies and improves the behavior of UDP sockets, which are now explicitly
opened per listen interface
3. transitions udp tracker, DHT and uTP socket manager over to using the new
udp sockets
4. greatly simplified udp_socket to only wrap a single underlying socket (as
opposed to one IPv4 and IPv6 socket)
5. improved behavior of bind-to-device
6. introduce an array_view type to make udp packet passing code simpler
7. simplify and make setting of DF flag more robust
8. simplify and regularize port mapping of listen sockets
Arvid Norberg