net: `Server.listen`, `Server.close` and `Socket.connect` return `this`

net: `Server.listen`, `Server.close` and `Socket.connect` return `this`

Just a syntactic sugar for doing, for example:

    var server = net.createServer(function (c) {
      c.end('goodbye, cruel world!\r\n');
      server.close().on('close', function () {
        console.log('really, goodbye!');
      });
    }).listen(1337);

Fixes #1922.