Make IPv4 and IPv6 listen on different sockets.

This commit implements support for listening on multiple sockets and
changes the default listen code to use one socket per address family (IPv4
and IPv6).

In addition, it changes the default listen binding for the RPC server to
localhost so only local clients can connect to it.

There need to be several options added to allow customization of these
settings and those will be in future commits.

Fixes #3.