Add pkgbase METALOG parse/check tool

metalog.lua is a script that reads METALOG file created by pkgbase
(make packages) and generates reports about the installed system
and issues.

This was developed as part of Yang's W2020 University of Waterloo co-
operative education term with the FreeBSD Foundation. kevans provided
some initial review; we will iterate on it in the tree.

Submitted by: Yang Wang <2333@outlook.jp>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24563