Method set_multicast_loopback

lib_acl_cpp/samples/multicast/main.cpp:8–10

	virtual ~multicast_thread() {}

	void set_multicast_loopback(bool on) {
		loopback_ = on;
	}

	bool open(const char* addr, const char* iface, int port) {
		unsigned oflags = acl::OPEN_FLAG_REUSEPORT;