acme: prompt for terms agreement

Allow Client.Register callers to report CA terms agreement,
simplifying the registration usage.

Change-Id: Ia2c237f31c8375b5ae669bbd07d9294c329f59b8
Reviewed-on: https://go-review.googlesource.com/23973
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed