* 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.

example/main/README.md

@@ -7,6 +7,7 @@
   * [Streams](#streams)
   * [RPC Operations](#rpc-operations)
   * [State data](#state-data)
+  * [Extensions](#extension)
   * [Authentication and NACM](#authentication-and-nacm)
   * [Systemd](#systemd)
   * [Docker](#docker)
@@ -270,6 +271,33 @@ The example contains some stubs for authorization according to [RFC8341(NACM)](h
 * A basic auth HTTP callback, see: example_restconf_credentials() containing three example users: andy, wilma, and guest, according to the examples in Appendix A in [RFC8341](https://tools.ietf.org/html/rfc8341).
 * A NACM backend plugin reporting the mandatory NACM state variables.
 
+## Extensions
+
+Clixon supports Yang extensions, but you need to write plugin code.
+The example backend implements an "example:e4" Yang extension, as follows:
+```
+    extension e4 {
+       description
+	   "The first child of the ex:e4 (unknown) statement is replaced with its first
+            child. This means that 'uses bar;' in the ex:e4 statement below is a valid
+            data node";
+       argument arg;
+    }
+    ex:e4 arg1{
+      uses bar;
+    }
+```
+
+The backend plugin code registers an extension callback in the init struct:
+```
+    .ca_extension=example_extension,        /* yang extensions */
+```
+
+The callback then receives a callback on all "unknown" Yang statements
+during yang parsing. If the extension matches "example:e4", it applies
+the extension. In the example, it replaces the "ex:e4" statements with
+its first child, making it a proper yang statement.
+
 ## Systemd
 
 Example systemd files for backend and restconf daemons are found under the systemd directory. Install them under /etc/systemd/system for example.