schema REFACTOR rename argument member in extension definition

The argname better describes meaning of the data and avoids conflicts
with the argument member in extension instances. The change affects both
the parsed and compiled extension definition structures (lysp_ext and
lysc_ext)
diff --git a/src/parser_yin.c b/src/parser_yin.c
index 5c9ae31..034142e 100644
--- a/src/parser_yin.c
+++ b/src/parser_yin.c
@@ -2071,7 +2071,7 @@
     LY_CHECK_RET(lyxml_ctx_next(ctx->xmlctx));
     LY_CHECK_RET(yin_parse_attribute(ctx, YIN_ARG_NAME, &ex->name, Y_IDENTIF_ARG, LY_STMT_EXTENSION));
 
-    struct yin_argument_meta arg_info = {&ex->flags, &ex->argument};
+    struct yin_argument_meta arg_info = {&ex->flags, &ex->argname};
     struct yin_subelement subelems[] = {
         {LY_STMT_ARGUMENT, &arg_info, YIN_SUBELEM_UNIQUE},
         {LY_STMT_DESCRIPTION, &ex->dsc, YIN_SUBELEM_UNIQUE},