Makes the following corrections to the "Applying Policy:" output:
- keep the last 1 snapshots snapshots => keep 1 latest snapshots
- keep the last 1 snapshots, 3 hourly, 5 yearly snapshots => keep 1 latest, 3 hourly, 5 yearly snapshots
Make restic forget --keep-within accept time ranges measured in hours and choose
accordingly which snapshots to keep and which to forget. Add relative tests.