* 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/restconf/restconf_methods.c

@@ -1674,7 +1674,7 @@ api_operations_post(clicon_handle h,
      */
     if ((ret = rpc_callback_call(h, xbot, cbret, r)) < 0)
 	goto done;
-    if (ret == 1){ /* Handled locally */
+    if (ret > 0){ /* Handled locally */
 	if (xml_parse_string(cbuf_get(cbret), NULL, &xret) < 0)
 	    goto done;
 	/* Local error: return it and quit */