subscriptions 6.x-1.0-rc1

This is Release Candidate 1 of the Drupal 6 port of Subscriptions 5.x-2.x.

It has been very well tested and we're confident that it is ready for release. Please give it one last close look and let us know within the next few days if you find any show stoppers. See #273505-23: Subscriptions for D6 is released! for details.

There have been only minor changes since BETA6, and that's why we're going right to RC1:

- Add a system option to append the mailkey in use to each mail message, for development/testing.
- Add a !has_files variable to help with conditional text for the !files variable.
- #259377: Add some new French strings, corrected plural formula.

There is one open issue, which we'll address after the release, because it will require significant testing: #350077: Maximum number of notification to send per cron job breaks Digest mode