* 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_plugin.h

@@ -67,8 +67,6 @@ typedef struct {
 /*
  * Prototypes
  */
-int backend_plugin_initiate(clicon_handle h); 
-
 int clixon_plugin_reset(clicon_handle h, char *db);
 
 int clixon_plugin_statedata(clicon_handle h, yang_stmt *yspec, cvec *nsc,