- Make create_tmp_table() set KEY_PART_INFO attributes for the keys it creates.
  This wasn't needed before but is needed now, when temp. tables that are 
  results of SJ-Materialization are being used for joins.
  This particular bug depended on HA_VAR_LENGTH_PART being set,
  but also added code to set HA_BLOB_PART and HA_NULL_PART when appropriate.