Fix monitor_event() to work at all.

There are three versions of monitor_event(), all taking
variadic arguments. The original code just has the first one
creating a va_list and passing that va_list variadically to
the second one... which creates a new va_list and passes it
variadically to the third one... and of course everything
blows up when we try to pull a non-va_list argument off the
stack.

The correct approach matches the C standard library's use
of printf/vprintf, scanf/vscanf, and so on. Once you make
a va_list, you must pass it only to functions which expect
a va_list parameter.

src/session_base.hpp

@@ -24,6 +24,7 @@
 #define __ZMQ_SESSION_BASE_HPP_INCLUDED__
 
 #include <string>
+#include <stdarg.h>
 
 #include "own.hpp"
 #include "io_object.hpp"
@@ -67,6 +68,7 @@ namespace zmq
         void terminated (zmq::pipe_t *pipe_);
 
         void monitor_event (int event_, ...);
+        void va_monitor_event (int event_, va_list args);
 
     protected: