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).

1 files changed, 13 insertions, 3 deletions

diff --git a/test/sys/mod.rs b/test/sys/mod.rs
index 3aab9fc5..31cf73b1 100644
--- a/test/sys/mod.rs
+++ b/test/sys/mod.rs
@@ -1,6 +1,15 @@
 mod test_signal;
-#[cfg(any(target_os = "freebsd", target_os = "dragonfly", target_os = "ios",
-          target_os = "netbsd", target_os = "macos", target_os = "linux"))]
+
+// NOTE: DragonFly lacks a kernel-level implementation of Posix AIO as of
+// this writing. There is an user-level implementation, but whether aio
+// works or not heavily depends on which pthread implementation is chosen
+// by the user at link time. For this reason we do not want to run aio test
+// cases on DragonFly.
+#[cfg(any(target_os = "freebsd",
+          target_os = "ios",
+          target_os = "linux",
+          target_os = "macos",
+          target_os = "netbsd"))]
 mod test_aio;
 #[cfg(target_os = "linux")]
 mod test_signalfd;
@@ -14,5 +23,6 @@ mod test_uio;
 #[cfg(target_os = "linux")]
 mod test_epoll;
 mod test_pthread;
-#[cfg(any(target_os = "linux", target_os = "android"))]
+#[cfg(any(target_os = "android",
+          target_os = "linux"))]
 mod test_ptrace;