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/link.rs

@@ -19,7 +19,7 @@ impl MacAddr {
     }
 }
 
-struct LinkFd(RawFd);
+pub(crate) struct LinkFd(RawFd);
 impl AsRawFd for LinkFd {
     fn as_raw_fd(&self) -> RawFd {
         self.0