The problem is caused because a DO statement is not following the
same handling procedure as a regular DML statement.
Among other things, we open and lock tables in the same operation,
whereas for regular DML statements, we open tables first, then
resolve the statement, then lock the statement. For DO statements
containing more than one expression, in particular those with
subqueries, it is a big problem that we do not follow the same phases
as for regular DML statements.

This particular problem is caused because closing a DO statement
is irregular.

The problem is solved by refactoring DO statement handling to execute
it like a SELECT statement. Thus, the following statement:

  DO expr1, expr2, ... exprN;

is executed as

  SELECT expr1, expr2, ... exprN FROM DUAL INTO /dev/null;

Instead of generating a list of expressions and evaluating them one
by one, we generate a SELECT statement where the expressions are
each evaluated exactly once. The difference compared to a regular
SELECT statement is that a specific Query_result object that does
not send back any results are generated.

There is one semantic change in this patch: DO statements do not
normally cause errors; they are instead converted to warnings.
In this patch, errors are generated as for regular SELECT statements
and returned to the client. Because of this, a few test results have
been changed to report errors.

Summary of changes:

- The function Item::evaluate() that evaluates an Item tree with
  type as given by field_type() is implemented.

- A new constructor for parse tree class PT_select_part2 is called
  when parsing a DO statement.

- PT_select constructor now takes an sql_command argument
  (for distinguishing between SELECT and DO).

- Preparing a DO statement is handled by mysql_test_select(),
  the same function as used for preparing SELECT.

- mysql_do() now creates a Query result subclass object and calls
  handle_query().

- The subclass Query_result_do has the following important functions:

  # ::send_data() calls Item::evaluate() but does not send any values.

  # ::send_eof() calls ::my_ok() in case of success in execution.

  # ::send_result_set_metadata() does nothing.