Problem: `ZMQ_MULTICAST_HOPS` doesn't affect `ZMQ_DGRAM` sockets

[khnsky]

It is impossible to change TTL of a DGRAM socket. Setting ZMQ_MULTICAST_HOPS doesn't affect ZMQ_DGRAM sockets because in zmq::udp_engine_t::plug only sets the TTL value if the socket is not raw (DGRAM is).

        if (!_options.raw_socket) {
            const ip_addr_t *out = udp_addr->target_addr ();
            _out_address = out->as_sockaddr ();
            _out_address_len = out->sockaddr_len ();

            if (out->is_multicast ()) {
                const bool is_ipv6 = (out->family () == AF_INET6);
                rc = rc
                     | set_udp_multicast_loop (_fd, is_ipv6,
                                               _options.multicast_loop);

                if (_options.multicast_hops > 0) {
                    rc = rc
                         | set_udp_multicast_ttl (_fd, is_ipv6,
                                                  _options.multicast_hops);
                }

                rc = rc | set_udp_multicast_iface (_fd, is_ipv6, udp_addr);
            }
        } else {

https://github.com/zeromq/libzmq/blob/master/src/udp_engine.cpp#L105-L124