Add dual IPv4/IPv6 stack support to the RPC server

The RPC server now listens for, and handles, incoming connections on
both IPv4 as well as IPv6.

If available (and usable) it uses a dual IPv4/IPv6 socket on systems
that support it (e.g. Linux and BSDs) and falls back to separate
IPv4/IPv6 sockets on systems that don't (e.g. Windows).

Signed-off-by: Giel van Schijndel <me@mortis.eu>