Extract the part of `p2p_private_broadcast.py` that configures and
starts the SOCKS5 server into a reusable function and put it into
`test_framework/socks5.py`.
Use bind port 0 to let the OS pick an available port instead of
hackishly assuming that `p2p_port(N)` is available where N is more
than the number of the nodes the test uses.
2ffa81fac4