Fix support for DragonFly

* DragonFly does not have a O_DSYNC flag
* Fix type_of_cmsg_data on DragonFly
* No fexecve() on DragonFly
* Do not run aio test cases on DragonFly
* Keep target lists in alphabetical order
* Unscrable #[cfg] directives and use cfg_if! macro instead
* Fix errno on DragonFly

Below follows an explanation why we have to use a C extension
to get errno working on DragonFly:

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

Tracking issue for `thread_local` stabilization:

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

Once this becomes stable, we can remove build.rs, src/errno_dragonfly.c,
remove the build-dependency from Cargo.toml, and use:

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

Now all targets will use the build.rs script, but only on DragonFly this
will do something. Also, there are no additional dependencies for
targets other than DragonFly (no gcc dep).

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