CLOEXEC on old Linux kernel.

[shirleyquirk]

If whatever is opening the socket is setting, CLOEXEC it doesn't expect to have 
to close the socket (see 
[here`](https://github.com/nim-lang/Nim/blob/927978345bbf57631828287850eda13b82277eb7/lib/pure/asyncdispatch.nim#L1585)
 so I don't think setting it to 0 will be enough