subselects into account

It is forbidden to use the SELECT INTO construction inside UNION statements
unless on the last SELECT of the union. The parser records whether it 
has seen INTO or not when parsing a UNION statement. But if the INTO was
legally used in an outer query, an error is thrown if UNION is seen in a
subquery. Fixed in 5.0 by remembering the nesting level of INTO tokens and 
mitigate the error unless it collides with the UNION.


mysql-test/r/union.result:
  Bug#32858: Test result
mysql-test/t/union.test:
  Bug#32858: Test case
sql/sql_class.cc:
  Bug#32858: Initializing new member
sql/sql_class.h:
  Bug#32858: Added property nest_level to select_result class.
sql/sql_yacc.yy:
  Bug#32858: The fix.