sysutils/arc_summary: New port

[jrm]

Summary

Print ZFS ARC cache statistics and other information

Sponsored by: The FreeBSD Foundation

Test Plan

poudriere testport and run-time test

[jrm]

• Do not create a subdirectory under distfiles

[diizzy]

Not a fan of using ? in a file name but that's a minor nitpick (I understand why you're doing it though)
Optional but since we're not using _REV anywhere except in MASTER_SITES perhaps consider use it directly and remove the variable?

LGTM