zsd (ZFS snapshot destroyer) is a zfs(8) wrapper to destroy snapshots
on a given dataset using a more convenient interface.

The number of snapshots to destroy can be specified directly, or
indirectly by specifying the number of snapshots that should be kept.

It goes nicely with zogftw's zogftw_snapshot_successfully_sent_hook()
to grow a certain number of snapshots on new datasets while keeping the
number of snapshots on old datasets constant.