Due to a copy/paste error, the COPYING file and subsequently created files
with the same content referred to the "Historical Permission Notice and
Disclaimer - sell variant", not the proper MIT license.

Replace with the proper MIT (Expat) license and add the use SPDX license
identifiers.

Acks below are from contributors with substantial changes, collected in MR !19
or via private email correspondence.

!19



Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Dorian Stoll <dorian.stoll@tmsp.io>
Acked-by: Jonas Ådahl <jadahl@gmail.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Acked-by: Evangelos Foutras <evangelos@foutrelis.com>
Acked-by: Niklas Haas <git@haasn.xyz>
Acked-by: Olivier Fourdan <ofourdan@redhat.com>
Acked-by: David Rosca <nowrep@gmail.com>
Acked-by: Lyude Paul <lyude@redhat.com>
Acked-by: Keith Packard <keithp@keithp.com>
Acked-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Acked-by: Michel Dänzer <michel@daenzer.net>
Acked-by: Tim Writer <tim.writer@amd.com>
Acked-by: Friedrich Schöller <code@schoeller.se>
Acked-by: Mikhail Konev <k.mvc@ya.ru>
Acked-by: Martin Pieuchot <mpi@openbsd.org>