Problem: ZMQ doesn't expose the MULTICAST_LOOP socket option

Solution: add a new ZMQ_MULTICAST_LOOP option for UDP sockets.

include/zmq.h

@@ -596,6 +596,7 @@ ZMQ_EXPORT void zmq_threadclose (void *thread);
 #define ZMQ_ZAP_ENFORCE_DOMAIN 93
 #define ZMQ_LOOPBACK_FASTPATH 94
 #define ZMQ_METADATA 95
+#define ZMQ_MULTICAST_LOOP 96
 
 /*  DRAFT 0MQ socket events and monitoring                                    */
 /*  Unspecified system errors during handshake. Event value is an errno.      */