This removes the code that might delete a Tahoe-LAFS nodedir and replaces it
with code that will try to save an unexpected copy of it out of the way
instead.

And test for (kind of) SSH availability

This gives us a good opportunity to mention the privacy policy, too.

At least, I suppose it was incorrect.  It won't build anywhere it has not
already built.  I don't know how it built where it already built.

The new version of PaymentServer has incompatible changes that require this.