Feature request: Option for last generation to never expire

[ekimekim]

I would like a way to specify that snapshots in the last generation should never be deleted, but kept indefinitely. Currently I can only do this via a workaround like giving P9999Y as the last interval, which is hacky. Ideally I'd like to be able to specify a special-case final interval as forever, which would indicate the final generation should have no end point.