[fuchsia] Config changes to build on Fuchsia

This patch disables various Unix-specific platform features that are
not enabled on Fuchsia. It also updates the mio version to 0.6.10,
which is the first release that supports Fuchsia.

src/net/tcp.rs

@@ -706,7 +706,7 @@ impl Future for TcpStreamNewState {
     }
 }
 
-#[cfg(unix)]
+#[cfg(all(unix, not(target_os = "fuchsia")))]
 mod sys {
     use std::os::unix::prelude::*;
     use super::{TcpStream, TcpListener};

src/net/udp/mod.rs

@@ -415,7 +415,7 @@ impl<T> Future for RecvDgram<T>
     }
 }
 
-#[cfg(unix)]
+#[cfg(all(unix, not(target_os = "fuchsia")))]
 mod sys {
     use std::os::unix::prelude::*;
     use super::UdpSocket;