[Libguestfs] using SPDX tags in libnbd and/or nbdkit?

As the topic has been coming up in other projects recently (for example, see https://gitlab.com/libvirt/libvirt-ci/-/merge_requests/39), should we update our files to either add SPDX tags, or even outright condense our existing copyright blurbs down to an SPDX tag plus documentation in a top-level file? The former (adding a line) is less problematic, the latter definitely requires buy-in from all contributors (although the list of contributors is relatively small, so it might be easy enough to get such buy-in). -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org