data tree CHANGE prepare updated validation flags

Except LYD_OPT_VAL_DATA_ONLY, none are implemented, only defined.
Operations (RPC/action/rpc-reply/notification) cannot be parsed
nor validated as they will have a separate function implemented.
Also, mandatory validation implemented.
diff --git a/src/tree_schema_helpers.c b/src/tree_schema_helpers.c
index 970b2e1..4bf97aa 100644
--- a/src/tree_schema_helpers.c
+++ b/src/tree_schema_helpers.c
@@ -1112,7 +1112,7 @@
     case LYS_ACTION:
         return "RPC/action";
     case LYS_NOTIF:
-        return "Notification";
+        return "notification";
     case LYS_USES:
         return "uses";
     default: