hi team!

Something to work on for a successor to the pending release:

I would like to introduce an option that allows libcurl to use HTTP/2 over TLS
but stick to HTTP/1.1 for plain tcp. Without built-in HTTP/2 support, it would
gracefully "downgrade" to just ask 1.1 even over TLS.

We add it as just another member in the enum for CURL_HTTP_VERSION_* numbers.
Like CURL_HTTP_VERSION_2_TLS_1_1_PLAIN or perhaps something shorter.

With such an option present in libcurl, we can then switch to letting that be
the default HTTP version selection in that and future libcurl versions.

It should be fairly low risk and applications could still override it like
today if they really need to stick to 1.1 over TLS even when capable to speak
HTTP/2.

As always, comments are welcome!

-- 
  / daniel.haxx.se
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