Patch for issue LIBZMQ-275. Dealing with VSM in distribution when pipes fail to write.

Signed-off-by: Ben Gray <ben@benjamg.com>

src/msg.hpp

@@ -68,6 +68,7 @@ namespace zmq
         void set_flags (unsigned char flags_);
         void reset_flags (unsigned char flags_);
         bool is_delimiter ();
+        bool is_vsm ();
 
         //  After calling this function you can copy the message in POD-style
         //  refs_ times. No need to call copy.