Enable O_DSYNC on FreeBSD with fcntl and aio_fsync (#2404)

It first appeared in FreeBSD 13.0.

changelog/2404.added.md

@@ -0,0 +1 @@
+`O_DSYNC` may now be used with `aio_fsync` and `fcntl` on FreeBSD.

src/fcntl.rs

@@ -114,7 +114,7 @@ libc_bitflags!(
         /// If the specified path isn't a directory, fail.
         O_DIRECTORY;
         /// Implicitly follow each `write()` with an `fdatasync()`.
-        #[cfg(any(linux_android, apple_targets, netbsdlike))]
+        #[cfg(any(linux_android, apple_targets, target_os = "freebsd", netbsdlike))]
         O_DSYNC;
         /// Error out if a file was not created.
         O_EXCL;

src/sys/aio.rs

@@ -55,6 +55,7 @@ libc_enum! {
         /// on supported operating systems only, do it like `fdatasync`
         #[cfg(any(apple_targets,
                   target_os = "linux",
+                  target_os = "freebsd",
                   netbsdlike))]
         O_DSYNC
     }