* YANG schema mount RFC 8528, Initial commit (work in progress)

* Keep track of YANG unknowns with ys_cvec of EXTENSION
* C-API: Init ys_cvec to NULL, added yang_cvec_add() and adjusted code to use it

lib/src/clixon_yang_internal.h

@@ -95,6 +95,7 @@ struct yang_stmt{
                                         Y_TYPE & identity: store all derived 
                                            types as <module>:<id> list
                                         Y_UNIQUE: vector of descendant schema node ids
+                                        Y_EXTENSION: vector of instantiated UNKNOWNSo
                                      */
     yang_type_cache   *ys_typecache; /* If ys_keyword==Y_TYPE, cache all typedef data except unions */
     char              *ys_when_xpath; /* Special conditional for a "when"-associated augment/uses xpath */