* Pushed tag to 4.0.1.PRE

* Restconf RFC 8040 increased feature compliance
  * Cache-Control: no-cache added in HTTP responses (RFC Section 5.5)
  * Restconf monitoring capabilities (RFC Section 9.1)
* Added support for Yang extensions
  * New plugin callback: ca_extension
  * Main backend example includes example code on how to implement a Yang extension in a plugin.
* JSON changes
  * Non-pretty-print output removed all extra spaces.
    * Example: `{"nacm-example:x": 42}` --> {"nacm-example:x":42}`
  * Empty JSON container changed from `null` to `{}`.
    * Empty list and leafs remain as `null`
* Removed unnecessary configure dependencies
  * libnsl, libcrypt, libm, if_vlan,...
* pseudo-plugin added, to enable callbacks also for main programs. Useful for extensions
* Yang Unique statements with multiple schema identifiers did not work on some platforms due to memory error.

apps/backend/backend_startup.c

@@ -138,9 +138,6 @@ startup_mode_startup(clicon_handle        h,
 	clicon_err(OE_FATAL, 0, "Invalid startup db: %s", db);
 	goto done;
     }
-    /* Load plugins and call plugin_init() */
-    if (backend_plugin_initiate(h) != 0) 
-	goto done;
     /* If startup does not exist, create it empty */
     if (xmldb_exists(h, db) != 1){ /* diff */
 	if (xmldb_create(h, db) < 0) /* diff */