Make license clearer

As you most probably know, there are many versions of the GPL license and many softwares are licensed under a specific version "or later".

The update-resolve-conf.sh scripts only reports "Licensed under the GNU GPL. See /usr/share/common-licenses/GPL." As I am using a FreeBSD machine, this file does not exists for me and I had to check on a linux machine to find that, today and on that particular machine, GPL is version 3, which does not prevent this file to be different on other machines or in the future, maybe if GPL v4 ever comes out.

Then it would be great if you could specify the version license once and for all in the script header, something like "Licensed under the GNU GPL version 3 or later. See /usr/share/common-licenses/GPL."

Thanks.