This patch comes after a proposal of mine at NFWS 2022 that resulted in
agreement to license recent .c files under GPLv2+ by the attendees at this
meeting:

- Stefano Brivio
- Fernando F. Mancera
- Phil Sutter
- Jozsef Kadlecsik
- Florian Westphal
- Laura Garcia
- Arturo Borrero
- Pablo Neira

It has already happened that one of the external library dependencies
was moved to GPLv3+ (libreadline), resulting in a change to libedit by
default in b4dded0c

 ("configure: default to libedit for cli").

I have added the GPLv2+ header to the following files:

                        Authors
                        -------
src/cmd.c               Pablo
src/fib.c               Florian
src/hash.c              Pablo
src/iface.c             Pablo
src/json.c              Phil + fixes from occasional contributors
src/libnftables.c       Eric Leblond and Phil
src/mergesort.c         Elise Lenion
src/misspell.c          Pablo
src/mnl.c               Pablo + fixes from occasional contributors
src/monitor.c           Arturo
src/numgen.c            Pablo
src/osf.c               Fernando
src/owner.c             Pablo
src/parser_json.c       Phil + fixes from occasional contributors
src/print.c             Phil
src/xfrm.c              Florian
src/xt.c                Pablo

Eric Leblond and Elise Lennion did not attend NFWS 2022, but they
acknowledged this license update already in the past when I proposed
this to them in private emails.

Update COPYING file too to refer that we are now moving towards GPLv2 or
any later.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>