Add support for RADIO/DISH sockets if draft API is enabled

This commit introduces new socket_type enumeration values as well
as the following supporting functions:

socket_t::join()
socket_t::leave()
message_t::group()
message_t::set_group()

tests/message.cpp

@@ -126,4 +126,11 @@ TEST(message, routing_id_persists)
     msg.set_routing_id(123);
     ASSERT_EQ(123u, msg.routing_id());
 }
+
+TEST(message, group_persists)
+{
+    zmq::message_t msg;
+    msg.set_group("mygroup");
+    ASSERT_STREQ("mygroup", msg.group());
+}
 #endif