Improvements to feature staging

This gets rid of the 'experimental' level, removes the non-staged_api
case (i.e. stability levels for out-of-tree crates), and lets the
staged_api attributes use 'unstable' and 'deprecated' lints.

This makes the transition period to the full feature staging design
a bit nicer.

src/libstd/sys/unix/ext.rs

@@ -29,7 +29,7 @@
 //! }
 //! ```
 
-#![experimental]
+#![unstable]
 
 use sys_common::AsInner;
 use libc;