Add REUSE/SPDX licensing/software BOM metadata

[felixc]

This adds machine-readable licensing information for all the files in the repository. This helps with auditing by distribution maintainers and corporate users who want to be careful about what software they bring in.

For trivial/uncopyrightable files, I've listed them under Creative Commons Zero, as recommended at https://reuse.software/faq/#uncopyrightable

For more info, see:

• https://reuse.software/
• https://spdx.dev/
• https://lwn.net/Articles/568286/