Promote certificate-related ctrl macros to functions. Also document them in the process. Almost done! BUG=404754 Change-Id: I3333c7e9ea6b4a4844f1cfd02bff8b5161b16143 Reviewed-on: https://boringssl-review.googlesource.com/5355 Reviewed-by: Adam Langley <agl@google.com>