Bug#16383173 WRONG COUNT FROM FOUND_ROWS() ON MYSQL 5.6

The handling of SQL_CALC_FOUND_ROWS in combination with
"order by with limit" was not specific enough,
and could lead to wrong results for FOUND_ROWS() 

The bug was that we took 'limit_found_rows' from filesort()
for cases where filesort() did not produce the final result.

Solution: let filesort() tell the execution machinery
that a priority queue was in fact used to filter the result.