New port: sysutils/zsm

A console program that manages ZFS snapshots.

• Manage multiple sets of rolling snapshots.
• Configure a custom schedule via the config file.
• Run in background by adding it to crontab.

Technically, most of the functionality is in zsm-lib,
and zsm is just a thin console interface.

WWW: https://zsm.rtfd.io/

PR: 234280
Submitted by: Matthias Lindvall <mattias.lindvall@gmail.com>