Remove non-standard Proxy-Connection header support

Almost certainly, nobody is sending it to us these days.

Remove all places where we send or inject it: i2ptunnel proxies and error pages, plus one place in streaming. In all places, we send Connection: close at the same time.
Remove most places where we look at the contents of the header; probably sufficient to just strip it in the proxies

Saves a few bytes, and e.g. if it's leaking out the outproxy, would be better not to.

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