* YANG when statement in conjunction with grouping/uses/augment

* Several cases were not implemented fully according to RFC 7950
    * Do not extend default values if when statements evaluate to false
    * Do not allow edit-config of nodes if when statements evaluate to false (Sec 8.3.2)
    * If a key leaf is defined in a grouping that is used in a list, the "uses" statement MUST NOT have a "when" statement. (See 7.21.5)
  * See [yang uses's substatement when has no effect #218](https://github.com/clicon/clixon/issues/2$

lib/clixon/clixon_xml.h

@@ -144,10 +144,10 @@ typedef enum yang_bind yang_bind;
 typedef struct xml cxobj; /* struct defined in clicon_xml.c */
 
 /*! Callback function type for xml_apply 
- * @retval    -1    Error, aborted at first error encounter
+ * @retval    -1    Error, aborted at first error encounter, return -1 to end user
  * @retval     0    OK, continue
- * @retval     1    Abort, dont continue with others
- * @retval     2    Locally, just abort this subtree, continue with others
+ * @retval     1    Abort, dont continue with others, return 1 to end user
+ * @retval     2    Locally abort this subtree, continue with others
  */
 typedef int (xml_applyfn_t)(cxobj *x, void *arg);
 

lib/clixon/clixon_yang.h

@@ -136,7 +136,7 @@ enum rfc_6020{
     Y_UNKNOWN,
     Y_USES,
     Y_VALUE,
-    Y_WHEN,
+    Y_WHEN, /* See also ys_when_xpath / ys_when_nsc */
     Y_YANG_VERSION,
     Y_YIN_ELEMENT,
     Y_SPEC  /* XXX: NOTE NOT YANG STATEMENT, reserved for top level spec */