set CHANGE replace LY_SET_OPT_USEASLIST by a single flag in ly_set_add()

There was just a single option and the ly_set API is quite stable for a
long time, so there will be hardly any new option. So instead of an
option bitmask with a single value, make the ly_set_add() parameter only
a single flag to switch off checking (and ignoring) items already
present in the set.
diff --git a/src/tree_data_helpers.c b/src/tree_data_helpers.c
index 8c4b310..7c03e49 100644
--- a/src/tree_data_helpers.c
+++ b/src/tree_data_helpers.c
@@ -244,7 +244,7 @@
             node->flags |= LYD_WHEN_TRUE;
         } else {
             /* remember we need to evaluate this node's when */
-            LY_CHECK_RET(ly_set_add(when_check, node, LY_SET_OPT_USEASLIST, NULL), );
+            LY_CHECK_RET(ly_set_add(when_check, node, 1, NULL), );
         }
     }