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/stream_engine.hpp

@@ -52,8 +52,8 @@ namespace zmq
     {
     public:
 
-        stream_engine_t (fd_t fd_, const options_t &options_,
-            bool as_server_, const std::string &endpoint);
+        stream_engine_t (fd_t fd_, const options_t &options_, 
+                         const std::string &endpoint);
         ~stream_engine_t ();
 
         //  i_engine interface implementation.