ci: consider testing on BSDs

[ee7]

Possible options:

1. GitHub Actions: macOS running BSD in a VM - VirtualBox is installed
2. sourcehut: Has FreeBSD and OpenBSD, but sourcehut will become a paid service when it enters beta.
3. Cirrus CI: FreeBSD only, but it seems supported to run other BSDs in a VM. Free for open source.
4. Something else.

FreeBSD can already run the Linux configlet release via the Linux binary compatibility, as documented here. You just need:

pkg install emulators/linux_base-c7
echo 'linux_enable="YES"' >> /etc/rc.conf`

When compiled natively, I can confirm that configlet works on FreeBSD, OpenBSD and NetBSD.

Related: #24

---

Low priority. I'm just creating some issues to document some things I'm aware of, but I have zero intention of doing (at least right now).