http: Add a new protocol for opening connections via http proxies

This opens a plain TCP connection through the proxy via the
CONNECT HTTP method. Normally, this is allowed for connections
on port 443, but can in general be used to allow connections
to any port (depending on proxy configuration), and could thus
be used to tunnel any TCP connection via a HTTP proxy.

Signed-off-by: Martin Storsjö <martin@martin.st>