Changed "db_multirow" to use "db_list_of_lists" instead of "db_list_of_ns_sets"
Thanks to the recent improvements in NaviServer for "db_list_of_lists", it is more efficient to use this in "db_multirow" (actually in "db_multirow_helper"). The implementation revealed a bug in "db_list_of_lists", when both options "-columns_var" and "-with_headers" were set.