std: Remove #[allow] directives in sys modules

These were suppressing lots of interesting warnings! Turns out there was also
quite a bit of dead code.

src/libstd/sys/unix/pipe.rs

@@ -143,7 +143,7 @@ impl UnixStream {
     fn lock_nonblocking<'a>(&'a self) -> Guard<'a> {
         let ret = Guard {
             fd: self.fd(),
-            guard: unsafe { self.inner.lock.lock().unwrap() },
+            guard: self.inner.lock.lock().unwrap(),
         };
         set_nonblocking(self.fd(), true);
         ret