dgram: refactor dgram to module.exports

Refactor dgram module to use the more efficient
module.exports = {} pattern.

PR-URL: nodejs#11696
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>

lib/dgram.js

@@ -57,7 +57,7 @@ function newHandle(type) {
 }
 
 
-exports._createSocketHandle = function(address, port, addressType, fd, flags) {
+function _createSocketHandle(address, port, addressType, fd, flags) {
   // Opening an existing fd is not supported for UDP handles.
   assert(typeof fd !== 'number' || fd < 0);
 
@@ -72,7 +72,7 @@ exports._createSocketHandle = function(address, port, addressType, fd, flags) {
   }
 
   return handle;
-};
+}
 
 
 function Socket(type, listener) {
@@ -99,12 +99,11 @@ function Socket(type, listener) {
     this.on('message', listener);
 }
 util.inherits(Socket, EventEmitter);
-exports.Socket = Socket;
 
 
-exports.createSocket = function(type, listener) {
+function createSocket(type, listener) {
   return new Socket(type, listener);
-};
+}
 
 
 function startListening(socket) {
@@ -561,3 +560,9 @@ Socket.prototype.unref = function() {
 
   return this;
 };
+
+module.exports = {
+  _createSocketHandle,
+  createSocket,
+  Socket
+};