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/printer_yin.c b/src/printer_yin.c
index 2604e9c..5ccea30 100644
--- a/src/printer_yin.c
+++ b/src/printer_yin.c
@@ -257,9 +257,9 @@
         yprp_extension_instances(ctx, LY_STMT_EXTENSION, 0, ext->exts, &flag, 0);
     }
 
-    if (ext->argument) {
+    if (ext->argname) {
         ypr_close_parent(ctx, &flag);
-        ypr_open(ctx, "argument", "name", ext->argument, flag2);
+        ypr_open(ctx, "argument", "name", ext->argname, flag2);
 
         LEVEL++;
         if (ext->exts) {
@@ -1273,11 +1273,11 @@
         ypr_close_parent(ctx, flag);
         inner_flag = 0;
 
-        if (ext_def->argument) {
+        if (ext_def->argname) {
             lysp_ext_instance_resolve_argument(ctx->module->ctx, &ext[u], ext_def);
         }
 
-        ypr_open(ctx, ext[u].name, (ext_def->flags & LYS_YINELEM_TRUE) ? NULL : ext_def->argument, ext[u].argument, inner_flag);
+        ypr_open(ctx, ext[u].name, (ext_def->flags & LYS_YINELEM_TRUE) ? NULL : ext_def->argname, ext[u].argument, inner_flag);
         LEVEL++;
         if (ext_def->flags & LYS_YINELEM_TRUE) {
             const char *prefix, *name, *id;
@@ -1288,9 +1288,9 @@
             /* we need to use the same namespace as for the extension instance element */
             id = ext[u].name;
             ly_parse_nodeid(&id, &prefix, &prefix_len, &name, &name_len);
-            ly_print_(ctx->out, "%*s<%.*s:%s>", INDENT, (int)prefix_len, prefix, ext_def->argument);
+            ly_print_(ctx->out, "%*s<%.*s:%s>", INDENT, (int)prefix_len, prefix, ext_def->argname);
             lyxml_dump_text(ctx->out, ext[u].argument, 0);
-            ly_print_(ctx->out, "</%.*s:%s>\n", (int)prefix_len, prefix, ext_def->argument);
+            ly_print_(ctx->out, "</%.*s:%s>\n", (int)prefix_len, prefix, ext_def->argname);
         }
         LY_LIST_FOR(ext[u].child, stmt) {
             if (stmt->flags & (LYS_YIN_ATTR | LYS_YIN_ARGUMENT)) {