iproute2 and libbpf: vendoring on the small scale

Mostly because it's wildly irregular and more or less impossible to extend. The ip output format is, uh, a bit painful to read (I suspect due to the lack of anything resembling punctuation), but it's extremely extensible and it does have one valuable extra property: each line-and-following-indented-lines group is in many cases valid or almost valid *input* to the corresponding ip subcommand, or at least similar enough that you can tell what to feed ip to replicate this configuration again (or to change it slightly). ifconfig has nothing like this.