patch 8.1.0519: cannot save and restore the tag stack

Problem:    Cannot save and restore the tag stack.
Solution:   Add gettagstack() and settagstack(). (Yegappan Lakshmanan,
            closes #3604)

src/alloc.h

@@ -18,5 +18,8 @@ typedef enum {
 	aid_qf_module,
 	aid_qf_errmsg,
 	aid_qf_pattern,
+	aid_tagstack_items,
+	aid_tagstack_from,
+	aid_tagstack_details,
 	aid_last
 } alloc_id_T;