Implement systemd packaging the Debian way

Upstream uses CMAKE hacks to inject the systemd files when systemd
is available. Those CMAKE hacks cannot work in pure Debian packaging
that is generated by debbuilder etc.

These changes were also made in MariaDB 10.2 in commit
https://github.com/MariaDB/server/commit/c912d05fab9db5829c403432fddece257c75ece0