new features:
set event_scheduler=ON|OFF will now try to init event scheduler
if it's not enabled
set event_scheduler=default will try to enable it based on
the value of the event_scheduler when mysqld was started
The problem was that wrong structure of mysql.event was not detected and
the server continued to use wrongly-structured data.
The fix is to check the structure of mysql.event after opening before
any use. That makes operations with events more strict -- some operations
that might work before throw errors now. That seems to be Ok.
Another side-effect of the patch is that if mysql.event is corrupted,
unrelated DROP DATABASE statements issue an SQL warning about inability
to open mysql.event table.
