- Detect if a table has field of type MYSQL_TYPE_VAR_STRING while running
   "CHECK TABLE t FOR UPGRADE" and indicate it need to be fixed
   with "REPAIR TABLE t".
 - When running a "REPAIR TABLE t" or "ALTER TABLE t FORCE" on the above
   table, install a special copy function to trim off the trailing spaces
   which we safely can say that the pre 5.0 mysqld didn't put there.