* YANG Leafref feature update

* Closer adherence to RFC 7950. Some of this is changed behavior, some is new feature.
  * Essentially instead of looking at the referring leaf, context is referred(target) node
  * Validation uses referred node
    * Validation changed to use type of referred node, instead of just "string"
  * Auto-cli
    * Changed to use type of referred node for typecheck
    * Completion uses referred node
  * Required instance / less strict validation
    * New: Leafrefs must refer to existing data leaf ONLY IF YANG `required-instance` is true
    * Previous: All leafrefs must refer to existing data leaf node
* Fixed: [Autocli does not offer completions for leafref to identityref #254](https://github.com/clicon/clixon/issues/254)

lib/src/clixon_yang_internal.h

@@ -80,6 +80,7 @@ struct yang_stmt{
 					leaf-list, 
 					config: boolean true or false
 					mandatory: boolean true or false
+					require-instance: true or false
 					fraction-digits for fraction-digits
 					unknown-stmt (optional argument)
 				     */