Conservative server rewrite

Some applications are hard to modify correctly.
Listening to multiple sockets is sometimes hard.

Listen on a single socket.
Specify an appropriate address family by a command line option.
To support both IPv4 and IPv6
Run 2 daemons ("foo -4" and "foo -6")
Some systems do not allow this.
Run a single AF_INET6 daemon assuming wildcard IPv6 bind(2) accepts an IPv4 connection.
Access control issue.