PollFd::revents should not return None

The change is trivial:

```
-    pub fn revents(&self) -> Option<PollFlags> {
-        PollFlags::from_bits(self.pollfd.revents)
+    pub fn revents(&self) -> PollFlags {
+        PollFlags::from_bits_retain(self.pollfd.revents)
     }
```

Motivation is that it is hard to handle `None` on the caller: suppose a flag I need is set, how one is supposed to handle `None`?