add net.Server.listenFD

Now that FD passing is in master, it'd be great to be able to use a received
socket (which has already had bind(2) and listen(2) called on it) to fire up a
new net.Server instance. This patch adds a net.Server.listenFD() method which
will start up the accept watcher on the provided FD.

lib/net.js

@@ -1167,11 +1167,24 @@ Server.prototype.listen = function () {
   }
 };
 
-Server.prototype._doListen = function () {
-  listen(this.fd, 128);
+Server.prototype.listenFD = function (fd) {
+  if (this.fd) {
+    throw new Error('Server already opened');
+  }
+
+  this.fd = fd;
+  this._startWatcher();
+};
+
+Server.prototype._startWatcher = function () {
   this.watcher.set(this.fd, true, false);
   this.watcher.start();
   this.emit("listening");
+};
+
+Server.prototype._doListen = function () {
+  listen(this.fd, 128);
+  this._startWatcher();
 }