Support HTTP CONNECT tunnels as an alternative to SOCKS

Note: This is NOT a ticket about adding a caching or rewriting or privacy-enhancing HTTP proxy to Tor.

We may want to support the "HTTP CONNECT" command as an alternative to SOCKS, since a fair number of applications support the former but not the latter. For the current definition of HTTP CONNECT, see RFC 7231.

Our existing HTTP parsing code should be sufficient to handle these requests, though we would want to make sure we got the semantics right.

The flexibility of HTTP could make this an alternative to proposal 229.

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