support 'parallel' updates and rallback of whole statement in case of error in...

support 'parallel' updates and rallback of whole statement in case of error in evalueting value which should be assigned
(BUG#1484)


mysql-test/r/variables.result:
  test of swaping variables
mysql-test/t/variables.test:
  test of swaping variables
sql/item_func.cc:
  new method of user variables to pre-fetch value
sql/item_func.h:
  new method of user variables to pre-fetch value
sql/set_var.cc:
  check() methods fetch & store value
sql/set_var.h:
  check() methods fetch & store value