Describe the feature you'd like to add to nginx

Add the __Host- prefix to all cookies sent to a client, and strip it from all cookies sent back to the server.

Describe the problem this feature solves

The __Host- prefix is an incredibly useful tool to isolate subdomains of a registered domain from each other. Due to browser implementation limitations, it is not as good as separate registered domains, but it is still a big win. However, not all servers use this prefix. This would make its use completely transparent to upstream servers.

Additional context

It might be possible to do this using a scripting language, but that is not obvious.