Allow LIMIT clause after DUAL.


mysql-test/r/limit.result:
  Added test result for BUG#8023.
mysql-test/t/limit.test:
  Added test for BUG#8023.
sql/sql_yacc.yy:
  Allow the specification of a LIMIT clause after DUAL. This is needed for queries as:
  select a from t1 union all select 1 from dual limit 1;
  In this query LIMIT is applied to the whole UNION, so it makes sense, however, the
  current parser did not allow any clause after DUAL.