Auto merge of #1085 - jmmv:freebsd-utimensat, r=alexcrichton

Expose futimens and utimensat on FreeBSD and DragonFly

Should have sent this out with #1084 but I didn't notice then that these were missing (see https://github.com/nix-rust/nix/pull/944).

Haven't been able to test these locally as I do not have access to these platforms. Will rely on CI to verify these for me.