Please support keybox files with kbxutil

This bug was originally reported by Daniel Kahn Gillmor (dkg@fifthhorseman.net) in Debian bug #871244:

keybox files are a file format specific to the GnuPG project.

The gnupg-utils package (currently only in experimental, but hopefully
soon to be moved to unstable) ships kbxutil, which should provide
sufficient textual diffs to get a better hint at the changes.

in debian stable, kbxutil used to be shipped in the gnupg package
itself, but depending on gnupg itself will pull in the whole suite
(including per-user daemons like gpg-agent and dirmngr, which aren't
necessary for the public data manipulation needed by diffoscope), so
gnupg-utils is probably a better way to go.