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_yang.c b/src/parser_yang.c
index a3b5c03..e2a749e 100644
--- a/src/parser_yang.c
+++ b/src/parser_yang.c
@@ -3637,7 +3637,7 @@
             LY_CHECK_RET(parse_status(ctx, &ex->flags, &ex->exts));
             break;
         case LY_STMT_ARGUMENT:
-            LY_CHECK_RET(parse_argument(ctx, &ex->argument, &ex->flags, &ex->exts));
+            LY_CHECK_RET(parse_argument(ctx, &ex->argname, &ex->flags, &ex->exts));
             break;
         case LY_STMT_EXTENSION_INSTANCE:
             LY_CHECK_RET(parse_ext(ctx, word, word_len, LY_STMT_EXTENSION, 0, &ex->exts));