dgram: scope redeclared variables

A few variables in `lib/dgram.js` are redeclared with `var` in a scope
where they have already been declared. These instances can be scoped
more narrowly with `const`, so that's what this change does.

lib/dgram.js

@@ -40,13 +40,13 @@ function lookup6(address, callback) {
 
 function newHandle(type) {
  if (type == 'udp4') {
- var handle = new UDP();
+ const handle = new UDP();
  handle.lookup = lookup4;
  return handle;
  }
 
  if (type == 'udp6') {
- var handle = new UDP();
+ const handle = new UDP();
  handle.lookup = lookup6;
  handle.bind = handle.bind6;
  handle.send = handle.send6;
@@ -209,7 +209,7 @@ Socket.prototype.bind = function(port_ /*, address, callback*/) {
  if (!self._handle)
  return; // handle has been closed in the mean time
 
- var err = self._handle.bind(ip, port || 0, flags);
+ const err = self._handle.bind(ip, port || 0, flags);
  if (err) {
  var ex = exceptionWithHostPort(err, 'bind', ip, port);
  self.emit('error', ex);
@@ -332,7 +332,7 @@ Socket.prototype.send = function(buffer,
  !!callback);
  if (err && callback) {
  // don't emit as error, dgram_legacy.js compatibility
- var ex = exceptionWithHostPort(err, 'send', address, port);
+ const ex = exceptionWithHostPort(err, 'send', address, port);
  process.nextTick(callback, ex);
  }
  }