More graceful handling of Subscriber Agreement changes

[Patches]

The Let's Encrypt Subscriber Agreement contains a clause that says you agree to any future changes they may make. Because of this, Let's Encrypt does not use this feature of the ACME protocol that requires you agree to a new Subscriber Agreement when it is changed.

@jsha

As @josh notes, we’ll be making a very minor update to our subscriber agreement on November 15, 2017. This should be transparent to almost all API users. You do not need to re-agree to the new subscriber agreement.

So the feature you want already exists. . But...

@jsha

Last time we updated the subscriber agreement, there were a handful of clients that tried to explicitly re-agree once we updated the subscriber agreement URL, producing errors. The ACME spec has been clarified to indicate that’s not necessary, and I believe the affected clients have been fixed.

And this is a known issue with acmetool. Hopefully they will fix it by the next update to the agreement.

[edited because I found the bug already existed after posting]