Add periodic script to backup zfs and zpool information

[rew]

Summary

Added a periodic script to create backup of output generated from zfs list, zpool list, zpool get
and zfs get.
Variables (with default values set) have been added to periodic.conf to customize output of the
respective commands.
The variables have been documentated in periodic.conf(5).

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=86388

[allanjude]

Approved By: allanjude (mentor)

[rew]

Updating D25638: Add periodic script to backup zfs and zpool information

Changed wording & macros used in periodic.conf(5) to reflect suggested changes by @bcr and @0mp
from a separate review, https://reviews.freebsd.org/D25631.

Updated Makefile to actually install the new periodic script.

Changed periodic script to be disabled by default.

Will bump .Dd on commit.

[0mp]

The manual page seems fine!