MDEV-16325 CREATE..SELECT..UNION creates a wrong field type for old varchar

- Adding Type_handler::traditional_merge_field_type()
- Removing real_type_to_type(), field_merge_type()
- Making Type_handler_var_string to merge as VARCHAR
- Additionally, fixing Field_string::print() to add the "/*old*/"
  comment into the data type for the old VARCHAR.
  This is similar to what MDEV-8267 earlier did for old DECIMAL.
- Adding tests

mysql-test/main/varbinary.result

@@ -82,8 +82,8 @@ drop table t1;
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `a` varbinary(255) DEFAULT NULL,
-  `b` varchar(255) DEFAULT NULL
+  `a` varbinary(255)/*old*/ DEFAULT NULL,
+  `b` varchar(255)/*old*/ DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 select length(a), length(b) from t1;
 length(a)	length(b)