This will also enable us in the future to collect statistics for
writes to internal tmp tables.

sql/handler.h:
  Added ha_write_tmp_row()
sql/opt_subselect.cc:
  ha_write_row -> ha_write_tmp_row
sql/sql_class.h:
  Added ha_write_tmp_row()
sql/sql_select.cc:
  ha_write_row -> ha_write_tmp_row