iproute2 and libbpf: vendoring on the small scale

You can see this effect when trying to make useful CI Docker images for Windows builds. I believe that Windows Docker containers are limited to around 20G by default (almost certainly by Docker Hub). I don't know how changeable this is (e.g., it might be baked in at (base) image creation time). Fun things with Windows container usages:

- Base images are usually 4GB+ (the Linux image with "all possible external dependencies" is 2.6GB)
- Visual Studio requires a specific base image that is 6GB+ which contains a .NET Runtime install which apparently can't be done in any other image (so much for composability)
- Installing one Visual Studio version and toolchain is another 3GB+
- Dependencies (e.g., Qt) can be another GB or more

This leaves you with very limited space for doing what you'd want inside of a container (such as a CI build). We're still using bare metal for Windows builds despite the existence of Docker on Windows.