applied the new function is_union() to places
  in the code that check the same condition.
- 5.0->5.1 merge fixes


mysql-test/r/subselect3.result:
  merge 5.0->5.1 : updated explain.
sql/item_subselect.cc:
  addendum of the fix for bug 27786:
  applied the new function is_union() to the applicable places.
sql/sql_derived.cc:
  addendum of the fix for bug 27786:
  applied the new function is_union() to the applicable places.
sql/sql_parse.cc:
  addendum of the fix for bug 27786:
  applied the new function is_union() to the applicable places.
sql/sql_select.cc:
  addendum of the fix for bug 27786:
  applied the new function is_union() to places 
  in the code that check the same condition.
sql/sql_union.cc:
  addendum of the fix for bug 27786:
  applied the new function is_union() to the applicable places.
sql/sql_view.cc:
  addendum of the fix for bug 27786:
  applied the new function is_union() to the applicable places.
sql/sql_yacc.yy:
  addendum of the fix for bug 27786:
  applied the new function is_union() to the applicable places.