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_connecter.cpp

@@ -112,7 +112,8 @@ void zmq::ipc_connecter_t::out_event ()
         return;
     }
     //  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, false, endpoint);
     alloc_assert (engine);
 
     //  Attach the engine to the corresponding session object.