Lower listen backlog to fix accept crashes

See https://github.com/Meziu/rust-horizon/pull/1

library/std/src/sys_common/net.rs

@@ -398,13 +398,20 @@ pub fn bind(addr: io::Result<&SocketAddr>) -> io::Result<TcpListener> {
         let (addrp, len) = addr.into_inner();
         cvt(unsafe { c::bind(sock.as_raw(), addrp, len as _) })?;
 
-        // Start listening
-        #[cfg(not(target_os = "horizon"))]
-        cvt(unsafe { c::listen(sock.as_raw(), 128) })?;
-        // 40 is the maximum for Horizon OS
-        #[cfg(target_os = "horizon")]
-        cvt(unsafe { c::listen(sock.as_raw(), 40) })?;
+        cfg_if::cfg_if! {
+            if #[cfg(target_os = "horizon")] {
+                // The 3DS doesn't support a big connection backlog. Sometimes
+                // it allows up to about 37, but other times it doesn't even
+                // accept 32. There may be a global limitation causing this.
+                let backlog = 20;
+            } else {
+                // The default for all other platforms
+                let backlog = 128;
+            }
+        }
 
+        // Start listening
+        cvt(unsafe { c::listen(sock.as_raw(), backlog) })?;
         Ok(TcpListener { inner: sock })
     }