Improve and extend use of bindgen

[FelixMcFelix]

Currently, we have a mixture of binding styles in OPTE. We have handspun bindings in e.g., illumos_sys_hdrs and xde::{dls, mac_sys, secpolicy, sys}, while the bindgen'd xde::ip is manually triggered.

While illumos header definitions are fairly stable, I'd rather it be the case that a) all *_sys type definitions are automatically updated for the target system, and b) extra kernel APIs we need to make use of (mac_link_* and friends) are minimal friction to add and that we can be certain of correctness. On point a) in particular, we should really be maximising the use of build.rs-managed _sys crates.

Once complete, these might be good to remove from the repo to help support future kernel/driver work in illumos.

[rcgoodfellow]

Bingen can have some pretty major pitfalls. Something worth considering as an alternative is ctf-bindgen. It's still very much at the "prototype an idea and see how it goes" phase, but worth thinking about as a future direction.