Bump the FreeBSD image used on Cirrus-CI to FreeBSD-13-2

Cirrus CI is retiring its Intel-based MacOS VMs in favour of using M1 VMs which means a newer image.

The gcc container is useful to quickly run compilation checks within it. However, this is currently now behind where it used to be, and as such, certain packages cannot be installed. We already compile got against Ubuntu/Alpine. That should be good enough.

This is now required to run gotwebd.

Whenever a CI change is made, also run the test suites. This is enabled for just Ubuntu and FreeBSD for now.

Add Ubuntu as a dedicated CI environment. By default, sh is aliase to dash which often provides interesting challenges for portability, so making this work here is a good thing as mistakes will be spotted earlier.

Apline doesn't like the use of cdefs.h

Add Alpine Linux to the list of CI checks. Alpine builds against muscl rather than glibc by default, and there's been a few compilation differenes to warrant having this distribution as a separate CI check.

Enable CI builds for MaxcOS on CirrusCI.

Cirrus-CI allows for FreeBSD builds as well as Linux.