Add shorthands for creating listening sockets

These a of particular use when you e.g. receive socket address to listen from
downstream code or from configuration file. They also make UDP and TCP API look
a bit more similar to what stdlib provides and reduce typing for very common
cases. 3 methods added:
- SockAddr.family() get AddressFamily of the address
- UdpSocket::new_bound() create listening UDP socket similar to stdlib
  UdpSocket::bind()
- TcpAcceptor::new() create listening TCP socket hiding intermediate steps