implement set_tcp_keepalive for linux

why use dummy implementation on linux?

Author: dovahcrow

src/libstd/sys/common/net2.rs

@@ -207,10 +207,16 @@ impl TcpStream {
         setsockopt(&self.inner, libc::IPPROTO_TCP, libc::TCP_KEEPIDLE,
                    seconds as c_int)
     }
+    #[cfg(target_os = "linux")]
+    fn set_tcp_keepalive(&self, seconds: u32) -> io::Result<()> {
+        setsockopt(&self.inner, libc::IPPROTO_TCP, libc::TCP_KEEPIDLE,
+                   seconds as c_int)
+    }
     #[cfg(not(any(target_os = "macos",
                   target_os = "ios",
                   target_os = "freebsd",
-                  target_os = "dragonfly")))]
+                  target_os = "dragonfly",
+                  target_os = "linux")))]
     fn set_tcp_keepalive(&self, _seconds: u32) -> io::Result<()> {
         Ok(())
     }