Merge change from MySQL AB:

ChangeSet@1.2557, 2007-11-28 19:43:50+01:00, tnurnberg@mysql.com +21 -0
  Bug#31177: Server variables can't be set to their current values

  Default values of variables were not subject to upper/lower bounds
  and step, while setting variables was. Bounds and step are also
  applied to defaults now; defaults are corrected quietly, values
  given by the user are corrected, and a correction-warning is thrown
  as needed. Lastly, very large values could wrap around, starting
  from 0 again. They are bounded at the maximum value for the
  respective data-type now if no lower maximum is specified in the
  variable's definition.

  mysql-test/r/innodb.result@1.171, 2007-11-28 19:43:48+01:00, tnurnberg@mysql.com +4 -0
    We throw a warning now when we adjust out of range parameters.