* 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)

test/test_netconf.sh

@@ -39,8 +39,7 @@ if [ $? -ne 0 ]; then
     err
 fi
 
-new "netconf tests"
-
+new "netconf get-config"
 expecteof "$clixon_netconf -qf $cfg" '<rpc message-id="101"><get-config><source><candidate/></source></get-config></rpc>]]>]]>' '^<rpc-reply message-id="101"><data/></rpc-reply>]]>]]>$'
 
 new "Add subtree eth/0/0 using none which should not change anything"