Add AsFd impl for AutoCloseFd

Will be used to remove RawFd usages.

src/fds.rs

@@ -106,6 +106,12 @@ fn as_raw_fd(&self) -> RawFd {
     }
 }
 
+impl AsFd for AutoCloseFd {
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        unsafe { BorrowedFd::borrow_raw(self.fd()) }
+    }
+}
+
 impl Default for AutoCloseFd {
     fn default() -> AutoCloseFd {
         AutoCloseFd { fd_: -1 }