plain_mechanism now uses options.as_server

- we need to switch to PLAIN according to options.mechanism
- we need to catch case when both peers are as-server (or neither is)
- and to use username/password from options, for client

src/tcp_connecter.cpp

@@ -126,7 +126,7 @@ void zmq::tcp_connecter_t::out_event ()
 
     //  Create the engine object for this connection.
     stream_engine_t *engine = new (std::nothrow)
-        stream_engine_t (fd, options, false, endpoint);
+        stream_engine_t (fd, options, endpoint);
     alloc_assert (engine);
 
     //  Attach the engine to the corresponding session object.