an nv(9) configuration interface for bridge(4): https://git.sr.ht/~lw/freebsd-src/commit/b2c96fb1c9f00314cdc4e8b70c554ecf257539e4
why?
- return actual error messages instead of just EINVAL
- don't break the ABI every time we modify struct ifbreq
- get or set multiple things in a single syscall
- allow more complex set requests, e.g. adding a new member and configuring its vlan access at the same time