This structure is used with setsockopt in BGP speaker implementations.

For more information, please see:
https://criticalindirection.com/2015/05/12/tcp_md5sig/.

Fixes golang/go#24830

Change-Id: Ic2137e843ad502dbe9a4227362b63164789c5877
Reviewed-on: https://go-review.googlesource.com/106656Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>