Identities re-introduced

However, the "durable socket" behaviour wasn't re-added.
Identities are used solely for routing in REQ/REP pattern.

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

src/msg.hpp

@@ -50,6 +50,7 @@ namespace zmq
         enum
         {
             more = 1,
+            identity = 64,
             shared = 128
         };