LICENSE: use GPLv2+, but note GPLv3 is preferred
In case a GPLv4 is released and I'm not around/alive to approve it,
it's probably better to let the FSF dictate the license than to be
stuck on a potentially ineffective (in the future) GPLv3.
Of course if the FSF changes the GPLv4 in a way I do not agree with,
the GPLv3 (which I currently agree with) remains valid for this
project. Currently the GPLv3 is sufficient for our needs, but
changes may come up in the future to ensure users continue to have
access to our code.