fix private type leak and dead_code warnings

- LinkFd -> pub(crate) so it's visible in return types of readable/writable
- #[allow(dead_code)] on Proto enum (Udp variant reserved for v2)

gatunad/src/upstream.rs

@@ -3,6 +3,7 @@
 use crate::frame::{UpstreamEntry, PROTO_TCP, PROTO_UDP};
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[allow(dead_code)]
 pub enum Proto {
     Tcp,
     Udp,