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/stream_engine.hpp

@@ -52,7 +52,8 @@ namespace zmq
     {
     public:
 
-        stream_engine_t (fd_t fd_, const options_t &options_, const std::string &endpoint);
+        stream_engine_t (fd_t fd_, const options_t &options_,
+            bool as_server_, const std::string &endpoint);
         ~stream_engine_t ();
 
         //  i_engine interface implementation.
@@ -112,6 +113,9 @@ namespace zmq
         //  Underlying socket.
         fd_t s;
 
+        //  True iff this is server's engine.
+        bool as_server;
+
         msg_t tx_msg;
 
         handle_t handle;