List Flag: auto-subscribe

Indicates to listmanager that anyone sending a submission to a list
should automatically be subscribed to that list (if the user isn't
already a subscriber).

If this flag is set, "invite-only" has no effect.

If this flag is set, and "check-addrs" is also set, and a submitter's
address fails the "check-addrs" tests, this flag is ignored for that
submission.
