Merge change from MySQL AB:

ChangeSet@1.2612, 2007-11-07 19:59:58+04:00, ramil@mysql.com +6 -0
  Fix for bug #26447: "ALTER TABLE .. ORDER" does not work with InnoDB
  and auto_increment keys

  Problems:
    1. ALTER TABLE ... ORDER BY... doesn't make sence if there's a
       user-defined clustered index in the table.
    2. using a secondary index is slower than using a clustered one
       for a table scan.

  Fixes:
    1. raise a warning.
    2. use the clustered index.

  mysql-test/r/innodb.result@1.203, 2007-11-07 19:59:56+04:00, ramil@mysql.com +15 -15
    Fix for bug #26447: "ALTER TABLE .. ORDER" does not work with InnoDB
    and auto_increment keys
      - results adjusted.