update.module doesn't implement hook_flush_caches()

While working on http://drupal.org/node/209242, I noticed that we don't flush the available update cache during update.php. That'd be another smart time to invalidate the cache, since whenever we're running update.php, it almost certainly means we installed a newer version of something, in which case, we want to check for available update data again.

Splitting this off into a separate issue/patch, since regardless of what you think about all the mtime business over at http://drupal.org/node/209242, this is still worth doing.