* 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)

apps/backend/clixon_backend_transaction.h

@@ -55,7 +55,6 @@ typedef int (*downcall_cb)(clicon_handle h, uint16_t op, uint16_t len,
  * (defined in config_dbdep.c)
  * @see transaction_data_t  internal structure
  */
-typedef void *transaction_data;
 uint64_t transaction_id(transaction_data td);
 void   *transaction_arg(transaction_data td);
 cxobj  *transaction_src(transaction_data td);