ci: consider testing on BSDs

Possible options:
1. GitHub Actions: macOS running BSD in a VM - [VirtualBox is installed](https://github.com/actions/virtual-environments/blob/82fa8f2880083cc7e310fb7f2aa912aa52438aa7/images/macos/macos-10.15-Readme.md)
1. [sourcehut](https://man.sr.ht/builds.sr.ht/compatibility.md): Has FreeBSD and OpenBSD, but sourcehut will become a paid service when it enters beta.
1. [Cirrus CI](https://cirrus-ci.org/features/): FreeBSD only, but it seems supported to run other BSDs in a VM. Free for open source.
1. Something else.

FreeBSD can already run the Linux `configlet` release via the Linux binary compatibility, as documented [here](https://docs.freebsd.org/en/books/handbook/linuxemu/). You just need:

```shell
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

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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).