Add FreeBSD to CI

This MR enables building and testing BIND on FreeBSD 11.3 and 12.0 from within GitLab CI.

For the time being, FreeBSD jobs are optional, i.e. need to be started manually for each pipeline and their failures do not cause the whole pipeline to fail. I think this would be a good approach for a burn-in period - the FreeBSD runner is based on GitLab Runner Custom Executor scripts I wrote and thus it is hard to tell how stable things are before developers try breaking stuff :-)

The only drawback of the scripts that I am currently aware of (compared to the Docker executor) is that Git repository contents are not cached between jobs and thus every job needs to clone a fresh copy of the whole BIND 9 Git repository. Since the FreeBSD runner is currently located in Silicon Valley, this only takes seconds and thus I do not expect it to be a problem. It is obviously possible to improve things, but at this point I am simply not convinced that the complexity introduced into the CI scripts by this change would be worth the benefit.