Shorten ZMQ_ROUTER_REASSIGN_IDENTITIES to ZMQ_ROUTER_HANDOVER. Also, added a test for the HANDOVER functionality.

src/router.hpp

@@ -118,7 +118,7 @@ namespace zmq
         // If true, the router will reassign an identity upon encountering a
         // name collision. The new pipe will take the identity, the old pipe
         // will be terminated.
-        bool reassign_identities;
+        bool handover;
 
         router_t (const router_t&);
         const router_t &operator = (const router_t&);