FreeBSD 13.2 support

[ydirson]

Recently-released FreeBSD 13.2 introduces Netlink support. libc has to expose the necessary bits for netlink-sys to build, for which I started rust-lang/libc#3201.

For netlink-packet-route as I understand it the problem is different: rtnl/constants.rs assumes that the many Linux constants to be available whatever the platform is. How would you like best to handle this?

Context: my goal is to get xen-guest-agent to work on FreeBSD.