Cleaned up option to force identity on outgoing connection

- renamed to ZMQ_CONNECT_RID
- fixed whitespace malformating around previous patch
- renamamed next_peer_id to next_rid in preparation for
  larger rename of IDENTITY to ROUTING_ID

Note: ZMQ_CONNECT_RID has no test case and no entry in the man
page, as yet.

src/socket_base.hpp

@@ -164,8 +164,10 @@ namespace zmq
 
         // Monitor socket cleanup
         void stop_monitor ();
+        
         // Next assigned name on a zmq_connect() call used by ROUTER and STREAM socket types
-        std::string next_identity;
+        std::string connect_rid;
+
     private:
         //  Creates new endpoint ID and adds the endpoint to the map.
         void add_endpoint (const char *addr_, own_t *endpoint_, pipe_t *pipe);