Netlink support for recent FreeBSD guests

Will avoid wasting cycles periodically checking for network changes, and will report them instantly.

This is a meta-ticket, as the code is ready here, and can be built using the relevant libc PR; it is waiting for support in libc, provided by the following PRs:

• on the libc main branch (for their v1.0):
  • Netlink support in libc for FreeBSD
  • API-breaking change in libc necessary for the former, which is scheduled for the next major release
• on the libc-0.2 branch, the same contents, but hiding the feature and API-breaking change behind a feature, to make it non-breaking by default: Freebsd Netlink support backported