support latest upstream and userspace implementation

Apologies for the delay in getting this started, but this is a large set of changes as a result of the work at https://github.com/tonarino/innernet.

It adds a number of what you'd consider "general cleanups" that take advantage of new features in bindgen, as well as more strict checks on things like interface names.

It also adds a userspace backend such that the library can be used to communicate with implementations like wireguard-go.

This has been in use for some time now with the innernet project, and I'd like to see if you were interested in including these changes in your crate. If you're not interested in actively maintaining it for now, I'm happy to be included as a co-maintainer to keep making improvements as well.

If this is too large of a change to include, I understand that as well and am happy to continue to maintain a fork.