Add a cirrus CI config file that runs the regression suite on linux amd64/aarch64 and on freebsd.

While there, use it in the tests too

We ended up copying too much data from the fastcgi process.

It's now possible to run only a subset of the tests with: ./runtime test1 test2 ...

The tests are still there, the suite is equivalent to the old one, but this one is better structured. The biggest annoyance I had with the old one was that it wasn't straightforward to test only a specific set of tests. It's still impossible, but it's way easier to do it now. This extract all the tests to their own functions. It's overall better in all possible regards.

libevent2 has this concept of "freezeness" of a buffer. It's a way to avoid accidentally write/remove data from the wrong "edge" of the buffer. The client_tls_{read,write} functions need to add/drain data from the opposite edge, hence the need for the unfreeze call. This is the minimum change in order to work on libevent2 too. Another way would be to define evbuffer_{un,}freeze as NOP on libevent 1, but it's ugly IMHO.

* add configure check * change the way the headers are required (copied from tmux)