Support 80-bit Onion Model In Perpetuity

As has been outlined by various persons on IRC, the email lists, onion services and even clearnet blogs, over the recent years ever since prop224 came to be proposed...

The current 80-bit model of onion addressing needs to be continued in perpetuity[1] as a legacy compatibility mode of onion addressing.

This mode has proven critical to users and communities in onionland that use OnionCat, OnionVPN, and other tools to achieve various communication models and application support, including interoperation with other networks, that is highly relavant to them.

It is expressly understood by this userbase that this legacy, perhaps even parallel, mode of operation will entail tradeoffs to prop224, including potentially enhanced anonymity risks, among other risks, and that code and bug support may be delayed vs prop224, and that those and other tradeoffs and issues are deemed acceptable to the legacy community.

[1] Or at least until a formal bi-directional resolution layer / registry / DHT or other mechanism is developed for prop224 (or any subsequent addressing proposal), for which in part, at least a deterministic 40+80 = resultant 128-bit IPV6 network stack interface compatible mode of operation is available that can supply what OnionCat/OnionVPN do today.