* CLICON_NACM_MODE config option, default is disabled.
* Added username attribute to all rpc:s from frontend to backend
* Added NACM backend module in example
* Backend RPC register callback function (Netconf RPC or restconf operation POST) has been changed from: `backend_rpc_cb_register()` to `rpc_callback_register()`
* Backend RPC callback signature has been changed from: `int cb(clicon_handle h, cxobj *xe, struct client_entry *ce, cbuf *cbret, void *arg)` has been changed to : `int cb(clicon_handle h, cxobj *xe, struct client_entry *ce, cbuf *cbret, void *arg)`
* Frontend netconf and restconf plugins can register callbacks as well with same API as backends.
