This adds a safety flag named `[node] reveal-IP-address`, for which the
default value is True. When this is set to False, any configuration that
might reveal the node's IP address (to servers, or the external network)
will cause a PrivacyError to be raised at startup, terminating the node
before it gets a chance to betray the user's privacy. It also adds docs
and tests.
refs ticket:1010