fixed mustiupdate subselect compatibility

(bug 217)



mysql-test/r/subselect.result:
  test of multi-update & subselect
mysql-test/t/subselect.test:
  test of multi-update & subselect
sql/sql_update.cc:
  added subselect compatibility to query cache
sql/sql_yacc.yy:
  right st_select_lex initialization 
  layout fix