* Restructure and more generic plugin API (cli,backend,restconf,netconf)

* For preparation for authorization RFC8341
  * Plugins add clixon_plugin_init() and api struct for function pointers, eg:
```
static const struct clixon_plugin_api api = {
    "example",
    clixon_plugin_init,
    ...
}
clixon_plugin_api *clixon_plugin_init(clicon_handle h)
{
    return (void*)&api;
}
```
  * Moved specific plugin functions from apps/ to generic functions in lib/
    * New generic plugin load function: clixon_plugins_load()
  * Removed client-local netconf plugins netconf_plugin_callbacks()
    * This was code used before generic YANG rpc calls
  * Added username to clixon handle:
    * clicon_username_get() / clicon_username_set()
  * Added authentication plugin callback
  * Removed some obscure plugin code that seem not to be used (please report if needed!)
    * CLI parse hook
    * CLICON_FIND_PLUGIN
    * clicon_valcb()
* Removed username to rpc calls (added below)

example/routing_cli.c

@@ -81,7 +81,7 @@ mycallback(clicon_handle h, cvec *cvv, cvec *argv)
 
     /* Show eth0 interfaces config using XPATH */
     if (clicon_rpc_get_config(h, "running","/interfaces/interface[name=eth0]",
-			      NULL, &xret) < 0)
+			      &xret) < 0)
 	goto done;
 
     xml_print(stdout, xret);