Problem: tests without ZAP handler are failing

Solution: emit events as expected by tests, and refuse connections when
ZAP is required but no handler started

Addresses #2711 partially

src/null_mechanism.hpp

@@ -55,13 +55,14 @@ namespace zmq
         virtual int zap_msg_available ();
         virtual status_t status () const;
 
+        bool zap_required () const;
+
     private:
 
         bool ready_command_sent;
         bool error_command_sent;
         bool ready_command_received;
         bool error_command_received;
-        bool zap_connected;
         bool zap_request_sent;
         bool zap_reply_received;