Hey

With this new argument to the CURLOPT_HTTP_VERSION option, libcurl is told to
ask for HTTP/2 only when using TLS and not over clear-text. There should be
very little risk or over-head from just asking over TLS so it can easily be
made the default for the curl tool when it uses a libcurl with HTTP/2 support.
Eventually we can probably make libcurl use this default too, when we've
test-run this for a good while with the curl tool.

Since HTTP/2 over HTTP adds to the request size and causes extra round-trips
when upgrading, it has a higher impact so I think we shouldn't have it by
default (at this point in time).

This mimics more or less what modern browsers do.

I'll land this really soon unless there are questions, doubts or objections?

-- 
  / daniel.haxx.se
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