Mark libc's `fs::Dir as Sync (#1232)

This is safe since all methods that mutate the internal state require
a &mut self. It also makes behavior consistent with the linux_raw
backend (whose `fs::Dir` is Sync+Send).

Fixes: #1230.

src/backend/libc/fs/dir.rs

@@ -227,11 +227,12 @@ impl Dir {
     }
 }
 
-/// `Dir` implements `Send` but not `Sync`, because we use `readdir` which is
-/// not guaranteed to be thread-safe. Users can wrap this in a `Mutex` if they
-/// need `Sync`, which is effectively what'd need to do to implement `Sync`
-/// ourselves.
+/// `Dir` is `Send` and `Sync`, because even though it contains internal
+/// state, all methods that modify the state require a `mut &self` and
+/// can therefore not be called concurrently. Calling them from different
+/// threads sequentially is fine.
 unsafe impl Send for Dir {}
+unsafe impl Sync for Dir {}
 
 impl Drop for Dir {
     #[inline]