[REUSE] Addressing some papercuts in the specification

[Drew DeVault]

Hi! I'm trying to adapt REUSE to my projects, but I find the per-file
headers to be a bit verbose and uncomfortable to read or write. It seems
that, on the spectrum from machine-readable to human-readable, these
have landed too far to the former.

I am instead using a modified approach in my projects, which, for
example, looks like so:

// License-Id: GPL-3.0-only
// Copyright: 2019 Drew DeVault <sir at cmpwn.com>
// Copyright: 2019 Haelwenn (lanodan) Monnier <contact at hacktivis.me>
// Copyright: 2019 Jeff Kaufman <jeff.t.kaufman at gmail.com>
// Copyright: 2019 Nate Dobbins <nated at posteo.net>
// Copyright: 2019 Noah Loomans <noah at noahloomans.com>
// Copyright: 2019 Philip K <philip at warpmail.net>
// Copyright: 2019 Simon Ser <contact at emersion.fr>
// Copyright: 2020 Drew DeVault <sir at cmpwn.com>
// Copyright: 2020 skuzzymiglet <skuzzymiglet at gmail.com>
// Copyright: 2021 Gianluca Arbezzano <ciao at gianarb.it>
// Copyright: 2021 sourque <contact at sourque.com>

Let's not get ahead of ourselves by prematurely over-namespacing
everything. The current license headers read like an SPDX advertisement.