It makes no sense to have a default tmp_table_size larger than the 
max_heap_table_size .  In usage, the tmp is ever limited to the max value, 
so I lowered the default tmp to the default max value.

A great idea would be to emit a warning when the tmp_table_size is set to 
greater than max_heap_table_size .


sql/mysqld.cc:
  Lowered tmp_table_size default to be the same as max_heap_table_size default.