OpenBSD: Fix keepalive_ms and set_keepalive_ms.

OpenBSD (as of relase 6.6) does not support the socket option TCP_KEEPIDLE.
To be using SO_KEEPALIVE instead made it compile but was not correct
and was failing at runtime.

NetBSD does support the option and should therefore be treated like generic
unix.  The use of SO_KEEPALIVE as the KEEPALIVE_OPTION on NetBSD was not
correct.

src/ext.rs

@@ -657,8 +657,9 @@ impl<T: AsRawSocket> AsSock for T {
 cfg_if! {
     if #[cfg(any(target_os = "macos", target_os = "ios"))] {
         use libc::TCP_KEEPALIVE as KEEPALIVE_OPTION;
-    } else if #[cfg(any(target_os = "openbsd", target_os = "netbsd"))] {
-        use libc::SO_KEEPALIVE as KEEPALIVE_OPTION;
+    } else if #[cfg(target_os = "openbsd")] {
+        // OpenBSD does not have a TCP_KEEPIDLE setsockopt (as of release 6.6).
+        // The sysctl variable net.inet.tcp.keepidle controls the setting globally.
     } else if #[cfg(unix)] {
         use libc::TCP_KEEPIDLE as KEEPALIVE_OPTION;
     } else if #[cfg(target_os = "redox")] {
@@ -731,9 +732,13 @@ impl TcpStreamExt for TcpStream {
     fn set_keepalive_ms(&self, keepalive: Option<u32>) -> io::Result<()> {
         try!(set_opt(self.as_sock(), SOL_SOCKET, SO_KEEPALIVE,
                     keepalive.is_some() as c_int));
-        if let Some(dur) = keepalive {
-            try!(set_opt(self.as_sock(), v(IPPROTO_TCP), KEEPALIVE_OPTION,
-                        (dur / 1000) as c_int));
+        cfg_if! {
+            if #[cfg(not(target_os = "openbsd"))] {
+                if let Some(dur) = keepalive {
+                    try!(set_opt(self.as_sock(), v(IPPROTO_TCP), KEEPALIVE_OPTION,
+                                (dur / 1000) as c_int));
+                }
+            }
         }
         Ok(())
     }
@@ -745,9 +750,15 @@ impl TcpStreamExt for TcpStream {
         if keepalive == 0 {
             return Ok(None)
         }
-        let secs = try!(get_opt::<c_int>(self.as_sock(), v(IPPROTO_TCP),
-                                        KEEPALIVE_OPTION));
-        Ok(Some((secs as u32) * 1000))
+        cfg_if! {
+            if #[cfg(target_os = "openbsd")] {
+                Ok(None)
+            } else {
+                let secs = try!(get_opt::<c_int>(self.as_sock(), v(IPPROTO_TCP),
+                                                KEEPALIVE_OPTION));
+                Ok(Some((secs as u32) * 1000))
+            }
+        }
     }
 
     #[cfg(windows)]