* Ensured you can add multiple callbacks for any RPC, including basic ones.

* Extra RPC:s will be called _after_ the basic ones.
  * One specific usecase is hook for `copy-config` (see [doc/ROADMAP.md] that can be implemented thus way.
* `rpc_callback_register` added a namespace parameter. Example:
   ```
     rpc_callback_register(h, empty_rpc, NULL, "urn:example:clixon", "empty");
   ```

apps/backend/backend_client.h

@@ -60,5 +60,6 @@ struct client_entry{
  */ 
 int backend_client_rm(clicon_handle h, struct client_entry *ce);
 int from_client(int fd, void *arg);
+int backend_rpc_init(clicon_handle h);
 
 #endif  /* _BACKEND_CLIENT_H_ */