http: overridable `clientError`

Make default `clientError` behavior (close socket immediately)
overridable. With this APIs it is possible to write a custom error
handler, and to send, for example, a 400 HTTP response.

    http.createServer(...).on('clientError', function(err, socket) {
      socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
      socket.destroy();
    });

Fix: #4543
PR-URL: https://github.com/nodejs/node/pull/4557


Reviewed-By: Brian White <mscdex@mscdex.net>