Consider to support Plan9

[bigsk05]

Now build constraints exclude all Go files in C:\Users\Bigsk\go\pkg\mod\github.com\valyala\fasthttp@v1.68.0\tcplisten, consider supporting Plan 9 here?

[erikdubbelboer]

Sure, a pull request with the required change is always welcome!

[bigsk05]

I will take the time to study this part, and it would be great if anyone else is interested and contributes