Implement ZMTP/3.0 PLAIN mechanism

This implements protocol handshake.
We still need to design and implement 1) API changes so a user
can set username and password, and 2) a mechanism for engine
to authenticate users.

src/ipc_listener.cpp

@@ -80,7 +80,8 @@ void zmq::ipc_listener_t::in_event ()
     }
 
     //  Create the engine object for this connection.
-    stream_engine_t *engine = new (std::nothrow) stream_engine_t (fd, options, endpoint);
+    stream_engine_t *engine = new (std::nothrow)
+        stream_engine_t (fd, options, true, endpoint);
     alloc_assert (engine);
 
     //  Choose I/O thread to run connecter in. Given that we are already