Update headers and COPYING

[Niols]

This PR:

• Moves headache and its files around. They don't need to be in src/ as they aren't really sources, and src/.headache.conf is included in headache's default configuration so it can go. This change also fixes a bug in which make headers would only affect files directly in src/ and not deeper.

• Updates both .header and COPYING to account for a copyright up to 2023 (instead of 2021 previously).

• Runs headache (via make headers) on all source files.

[Niols]

I'm moving this PR on top of #136 to have a proper CI check.