x/sys/unix: add tcp_md5sig structure on Linux

[mdlayher]

I'm working on adding tcp_md5sig to x/sys/unix for use in metallb/metallb#215.

https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/tcp.h#L253

This is a Linux socket option used with BGP speakers: https://criticalindirection.com/2015/05/12/tcp_md5sig/.

I've got a CL in progress but I'm not sure the generated code is quite right. Will submit it shortly for discussion.

[gopherbot]

Change https://golang.org/cl/106656 mentions this issue: unix: add tcp_md5sig on Linux