prepared statements when LIMIT is used" and post-review comments.
The fix changes the approach we calculate the need for ORDER BY 
in UNION: the previous was not PS friendly, as it damaged SELECT_LEX 
options in case of single select.


mysql-test/r/ps.result:
  Test results fixed: the test case for Bug#6088
mysql-test/r/subselect.result:
  Test results fixed: now we don't perform ORDER BY for parts of UNION
  if there is no LIMIT clause.
mysql-test/t/ps.test:
  A test case for Bug#6088 "FOUND_ROWS returns wrong values for
   prepared statements when LIMIT is used".
sql/sql_union.cc:
  The actual fix for Bug#6088:
  - don't modify SELECT_LEX'es
  - use boolean variable can_skip_order_by to check if we can skip
    ORDER BY in UNION