PostgreSQL incompatibility in rsvp.cron

The query in rsvp.cron is not PostgreSQL-compatible. The attached patch makes it PostgreSQL-compatible. Note, however, that the new query is not MySQL-compatible. I'm not sure that there's a Standard SQL way to do what is done in this query (MySQL allows multiple tables after the UPDATE keyword, PostgreSQL handles this by using FROM to specify additional tables). To handle both, you're going to need to detect which is in use, and use the appropriate query, but so far I haven't found a reliable way to do this.

For the moment, those using RSVP on a PostgreSQL-based Drupal site might want to apply this patch.