Allow for Unique keys to be returned with phpbb_db_tools::sql_list_index()

I've recently received some headache from this, mainly due to the fact we are trying to support unique indexes with UMIL, however when we use the table_index_exists() method from the DB tools, it will remove all unique indexes from the result to be returned. I suggest an optional, Boolean parameter for phpbb_db_tools::sql_list_index() to show the unique results as well.

phpbb_db_tools::sql_list_index($table_name, $show_unique = false)