Make SHOW EXPLAIN work for queries that do "Using temporary" and/or "Using filesort"
- Patch#1: Don't lose "Using temporary/filesort" in the SHOW EXPLAIN output.
- Implement new approach to testing (the DBUG_EXECUTE_IF variant)
- add an 'evalp' mysqltest command that is like 'eval' except that
it prints the original query.
- Fix select_describe() not to change join_tab[i]->type
- More tests
- Fix st_select_lex::set_explain_type() to allow producing exactly the
same EXPLAINs as it did before. SHOW EXPLAIN output may produce
select_type=SIMPLE instead or select_type=PRIMARY or vice versa (which
is ok because values of select_type weren't self-consistent in this
regard to begin with)
