* 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.

lib/src/clixon_yang_internal.h

@@ -82,8 +82,6 @@ struct yang_stmt{
     yang_stmt         *ys_mymodule;  /* Shortcut to "my" module. Augmented
 					nodes can belong to other 
 					modules than the ancestor module */
-
-    char              *ys_extra;     /* For unknown */
     cg_var            *ys_cv;        /* cligen variable. See ys_populate()
 					Following stmts have cv:s:
 				        leaf: for default value
@@ -91,7 +89,7 @@ struct yang_stmt{
 					config: boolean true or false
 					mandatory: boolean true or false
 					fraction-digits for fraction-digits
-					unknown-stmt (argument)
+					unknown-stmt (optional argument)
 				     */
     cvec              *ys_cvec;      /* List of stmt-specific variables 
 					Y_RANGE: range_min, range_max