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

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

portable: ci: disable gcc container 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.

portable: mention libevent dependency This is now required to run gotwebd.

portable: regress: add test stage to all runs Whenever a CI change is made, also run the test suites. This is enabled for just Ubuntu and FreeBSD for now.

portable: ci: add ubuntu 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.

portable: alpine: install bsd-compat-headers Apline doesn't like the use of cdefs.h

CI: add alpine linux 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.

portable: CI: add MacOS definition Enable CI builds for MaxcOS on CirrusCI.

Cirrus-CI: add build YAML file Cirrus-CI allows for FreeBSD builds as well as Linux.