UNION's bug fixes and code clean-up


mysql-test/r/subselect.result:
  An insignificant consequent of my UNION"s clean-up
sql/sql_derived.cc:
  Fix a bug that our CTO reported
sql/sql_lex.h:
  Fixing some bugs in UNION's with code clean-up
sql/sql_union.cc:
  Fixing some bugs in UNION's with code clean-up
sql/sql_yacc.yy:
  Fixing some bugs in UNION's with code clean-up