This adds RFC support to the existing methods which, in conjunction with the new order based methods implemented in golang.org/cl/192779, completes a Client capable of obtaining certificates from RFC compliant CAs. Updates golang/go#21081 Change-Id: I3aabc50928d3e4e49ee202eb6695135d5ad86821 Reviewed-on: https://go-review.googlesource.com/c/crypto/+/194379 Reviewed-by: Filippo Valsorda <filippo@golang.org>