LABEL flag added to the wire format

So far there was no distinction between message parts used by 0MQ
and message parts used by user. Now, the message parts used by 0MQ
are marked as 'LABEL'.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/msg.hpp

@@ -47,8 +47,9 @@ namespace zmq
         //  Mesage flags.
         enum
         {
-            more = 1,
-            shared = 128
+            label = 1,
+            shared = 64,
+            more = 128
         };
 
         bool check ();