* 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_xpath_ctx.c

@@ -90,7 +90,7 @@ ctx_free(xp_ctx *xc)
 xp_ctx *
 ctx_dup(xp_ctx *xc0)
 {
-    static xp_ctx *xc = NULL;
+    xp_ctx *xc = NULL;
     
     if ((xc = malloc(sizeof(*xc))) == NULL){
 	clicon_err(OE_UNIX, errno, "malloc");