Parse column names (and not variables) only in UPDATE ... SET ...


mysql-test/r/sp-error.result:
  New test case for BUG#2272
mysql-test/t/sp-error.test:
  New test case for BUG#2272
sql/sql_yacc.yy:
  "UPDATE table SET id = val" should only recognize column names, and not
  local SP variables for 'id'.
  (Also removed "as locator" syntax which is not supported.)