Remove $FreeBSD$: one-line lua tag

Remove /^--\s*\$FreeBSD\$.*$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

pkgbase: report error if files are installed multiple times

Files installed by `make installworld` should be installed only once.

PR: 244596
Sponsored by: The FreeBSD Foundation

pkgbase: hide duplicate METALOG directory warnings under verbose

Creating directories multiple times is an inherent side effect of the
way installation is done. Hide warnings from duplicate directory
entries (with identical metadata) under metalog_reader's verbose mode.

Duplicate file entries are always reported. They currently generate
warnings but will be switched to errors once the few instances currently
in the tree are fixed.

PR: 244596, 271178
Reviewed by: kevans
Sponsored By: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39898

pkgbase: report errors via metalog reader exit status

Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37482

pkgbase: report type for duplicated METALOG entries

Duplicate METALOG file entries are more of a concern than duplicate
directories. The metalog check tool previously did not include the
entry type in the warnings, making it hard to find the ones of concern.

Sponsored by: The FreeBSD Foundation

pkgbase: examine METALOG files relative to stage root directory

Previously we stripped the '.' from the beginning of each METALOG entry
to determine the path to stat. This meant that we examined files on the
build host, not the staged files.

Instead, strip off the last part of the specified METALOG pathname to
find the stage root directory, and stat files relative to that.

Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37412

pkgbase: Fix a typo in a source code comment

- s/begining/beginning/

MFC after: 3 days

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