licensing: relicense non-library code under GPL-2.0-or-later

licensing: relicense non-library code under GPL-2.0-or-later

LGPL-2.1 is a license meant for shared libraries. Because I didn't know
any better I used it for all files in the repository, including programs
linking against libgpiod. The standard approach for many similar projects
is to use LGPL for library code and GPL for programs (and also Makefiles
and other files containing code or configuration).

Relicense all programs, tests, makefiles and autotools files under
GPL-2.0 or later.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Kent Gibson <warthog618@gmail.com>