Future-proof the kevent ABI
Future-proof the kevent ABI

FreeBSD 12 changes struct kevent.  For now, libc always binds to the
11-compat ABI.  But that will change some day.  Adjust Nix's code to
build with either struct definition.

nix-rust/nix,asomers/nix,asomers/nix,nix-rust/nix