Bugfix: Support "unlimited" in `forget --keep-*` options

Restic would previously forget snapshots that should have been kept when a
negative value was passed to the `--keep-*` options. Negative values are now
forbidden. To keep all snapshots, the special value `unlimited` is now
supported. For example, `--keep-monthly unlimited` will keep all monthly
snapshots.

https://github.com/restic/restic/issues/2565
https://github.com/restic/restic/pull/4234
