This patch adds DEFAULT as a possible dynamic SQL parameter, e.g.:
  EXECUTE IMMEDIATE 'INSERT INTO t1 (column) VALUES(?)' USING DEFAULT;
  EXECUTE IMMEDIATE 'UPDATE t1 SET column=?' USING DEFAULT;
and for similar PREPARE..EXECUTE queries.

This is done for symmetry with the STMT_INDICATOR_DEFAULT indicator in
the client-server PS protocol.

The changes include:
- Allowing DEFAULT as a possible option in execute USING clause (sql_yacc.yy)
- Adding "virtual bool Item::save_in_param(THD *thd, Item_param *param)",
  because "normal" items (that have real values) and Item_default_value
  have now different actions when assigning itself as an Item_param value.
- Fixing switch() statements in a few Item_param methods not to have "default",
  because it was easy to forget to add a new "case" when adding a new XXX_VALUE
  value into the enum Item_param::enum_item_param_state.
  This is important, as we'll be adding new values soon, e.g. for MDEV-11359.
  Removing "default" helped to find and report bugs MDEV-11361 and MDEV-11362,
  because DECIMAL_VALUE is obviously not properly handled in some cases.