Clean up, finalize, and merge proposal 346

Proposal 346 extends and clarifies subprotocol versions. Parts of it are already in effect (like our interpretations of protovers as flags). Other parts are incomplete or wrong. We should finish the proposal correct it, and merge it into torspec.

It will be helpful for a lot of stuff related to future projects, including CGO. It will be especially helpful for converging features between hs-ntor and ntorv3.

To merge, by section:

• Semantics: Protocol capabilities are feature flags.
• Semantics: Foo=X is called a "capability".
• How to document: Never say "or later".
• Requesting an opt-in-feature (circuit extension)
• Making features that can be disabled
• Subprotocol capabilities in onion services