optimiser bug patch.


mysql-test/r/distinct.result:
  test results reverted
mysql-test/r/order_by.result:
  test results reverted
sql/sql_select.cc:
  found_constrain renamed to found_constraint
  We don't perform full cartesian product in 
  case when JOIN BUFFER is used: it was taken
  into account.
  s->read_time may contain range index read time,
  so to get full table scan time is necessary to
  call s->table->file->scan_time().