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/ctx.hpp

@@ -97,7 +97,8 @@ namespace zmq
 
         // Monitoring specific
         int monitor (zmq_monitor_fn *monitor_);
-        void monitor_event (zmq::socket_base_t *socket_, int event_, va_list args_);
+        void monitor_event (zmq::socket_base_t *socket_, int event_, ...);
+        void va_monitor_event (zmq::socket_base_t *socket_, int event_, va_list args_);
 
         enum {
             term_tid = 0,