improved mechanisn of detection posibility to be NULL for single row queries
switched off substitution optimisation for single row subqueries in PS due to problem in resolving substituted expressions
(changes to make subselects test working with PS protocol)


mysql-test/r/subselect.result:
  removed incorrect error message about aggregate functions
  improved mechanisn of detection posibility to be NULL for single row queries
mysql-test/t/subselect.test:
  removed incorrect error message about aggregate functions
sql/item_subselect.cc:
  removed incorrect error message about aggregate functions
  switched off substitution optimisation for single row subqueries in PS due to problem in resolving substituted expressions
  improved mechanisn of detection posibility to be NULL for single row queries
sql/item_subselect.h:
  new method to help in NULL ability detection