Fix errno on DragonFly

__dfly_error() was removed from Rust many years ago.

DragonFly uses a thread-local errno variable, but #[thread_local] is
feature-gated and not available in stable Rust as of this writing
(Rust 1.31.0). We have to use a C extension to access it.

Tracking issue for `thread_local` stabilization:

    https://github.com/rust-lang/rust/issues/29594

Once this becomes stable, we can simply use:

    extern { #[thread_local] static errno: c_int; }