Netlink support on FreeBSD 13.2+

[ydirson]

FreeBSD 13.2 (here x86_64-unknown-freebsd) gained the Netlink layer, which seems to be
supported today only for Linux-like and Fuschia.

Bindings at https://github.com/rust-netlink/netlink-sys need a couple of bits from libc to build, most notably PF_NETLINK, sockaddr_nl, SOL_NETLINK and friends.