Haiku: add definitions for the Haiku's native sytem API.

On the Haiku platform, the POSIX (and BSD) API coexists with the native API,
that has its origins on the BeOS platform. Unlike other UNIX-like platforms,
the native API is not an extension of the POSIX API, but instead exists sui
generis, and many of the POSIX concepts have their own native variety, with
relatively limited interoperability.

Nontheless, the native API coexists in the same library as the standard C and
POSIX functions, namely libroot.so, and therefore this crate is a good place
to add bindings to it.

This commit implements most of Haiku's support kit, the most important parts
of the kernel kit, and a part of the storage kit.

src/unix/haiku/mod.rs

@@ -1553,3 +1553,6 @@ cfg_if! {
         pub use self::b32::*;
     }
 }
+
+mod native;
+pub use self::native::*;