sysutils/arc_summary: New port

arc_summary is included in the OpenZFS repository. It provides basic
information on the ZFS ARC, its efficiency, the L2ARC (if present), the
Data Management Unit (DMU), Virtual Devices (VDEVs), and tunables. See
the in-source documentation and code at
https://github.com/openzfs/zfs/blob/master/module/zfs/arc.c for details.
The original introduction to arc_summary can be found at
http://cuddletech.com/?p=454.

Reviewed by: diizzy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35441