Based on RFC 8410.

Updates #25355

Change-Id: If7abb7eeb0ede10a9bb3d2004f2116e587c6207a
Reviewed-on: https://go-review.googlesource.com/c/go/+/175478
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>