Keep enums in sync with systemd

We enumerate the enums [here](https://github.com/systemd/systemd/blob/main/src/libsystemd/sd-network/network-util.h) in networkd.rs ... Wonder if we could write some weekly CI to do some C to rust interaction and check we're still in sync.

- Don't want to really have build time C to Rust fun ... plus including systemd header files.

Thoughts?