Merge: CI: macOS jobs for Gitlab CI

Add 3 CI jobs for macOS, equivalent to `build_tools`, `test_some` and `test_full_nitcs`. They are executed by a shell executor on a macOS host, Nit Gitlab repos should have a such a runner with the tag `macos`. The new jobs are similar enough to the GNU/Linux versions that they can share the same script.

This PR also fixes 3 problems with macOS portability:

* Workaround for the error with kqueue on libevent and nitcorn clients.

* Set `sed -E` explicitly in `check_manpages.sh` for portability with macOS.

* Download bdwgc in the CI docker image, to prevent `test_full_nitcs_macos` to fail on `hello_ios` because it downloads bdwgc at compilation and prints to the test output. This will require regenerating the official Docker image.

Pull-Request: #2743
Reviewed-by: Jean Privat <jean@pryen.org>