biboumi: init at 6.1 (#30193)

* biboumi: init at 6.1

TODO: integrate config in NixOS

* biboumi: remove external buildtime dep

- fetch catch.hpp in a reproducible way
- add maintainer
- enable tests
- remove git dep
- enable parallel building
- add pandoc dep for man page
- nitpicks

* biboumi: refine substitutions

- only CMakeLists.txt has to be patched regarding /etc/biboumi
- substitute /bin/kill in systemd service file
- prepare for configuring policy_directory in a future cfg file