net: improve arbitrary tcp socket support

Consider this example:

  // fd 3 is a bound tcp socket
  var s = net.createServer(cb);
  s.listen({ fd: 3 });
  console.log(s.address());  // prints null

This commit makes net.Server#address() print the actual address.

Ditto for non-listen sockets; properties like net.Socket#localAddress
and net.Socket#remoteAddress now return the correct value.

Fixes #5009.