dgram: make setMulticastTTL() conform to v0.4 API

- throw if the ttl argument is not a number
- return the ttl argument (not particulary useful but it's what v0.4 did)

Note that the 0 < ttl < 256 check has *not* been reinstated. On Linux, -1 is a
valid argument to setsockopt(IPPROTO_IP, IP_TTL).

lib/dgram.js

@@ -237,11 +237,15 @@ Socket.prototype.setTTL = function(arg) {
 
 
 Socket.prototype.setMulticastTTL = function(arg) {
-  if (this._handle.setMulticastTTL(arg) == -1) {
+  if (typeof arg !== 'number') {
+    throw new TypeError('Argument must be a number');
+  }
+
+  if (this._handle.setMulticastTTL(arg)) {
     throw errnoException(errno, 'setMulticastTTL');
   }
 
-  return true;
+  return arg;
 };