http: add CURLINFO_HTTP_VERSION and %{http_version}

[gevaerts]

Adds access to the effectively used http version to both libcurl and curl.

Initial attempt, almost certainly not final yet.

[jay]

I like this idea, I have read of it a few times, as recently as last month. What do you think about using the existing CURL_HTTP_VERSION_X_X enumerations for getinfo, and then for the tool something like:

verstring = ((ver == CURL_HTTP_VERSION_2_0) ? "2.0" :
             (ver == CURL_HTTP_VERSION_1_1) ? "1.1" :
             (ver == CURL_HTTP_VERSION_1_0) ? "1.0" : "0");

[gevaerts]

I've updated the patch to use CURL_HTTP_VERSION now.

[bagder]

👍 to merge first thing after the pending release

[bagder]

Note that the travis-ci test failure is because the new symbol is not properly added to docs/libcurl/symbols-in-versions

[gevaerts]

I've added it to symbols-in-versions now.

[bagder]

Ganesh Nikam just posted a version of this patch on the mailing list:

https://curl.haxx.se/mail/lib-2016-05/0131.html

He included a little example in his man page that could be merged into this patch to make it perfect I think!

[ganeshnikam18]

I think the string "2.0" would be better here instead of "2".

[gevaerts]

The consensus seems to be that it's http 2, not 2.0. See e.g. https://http2.github.io/faq/#is-it-http20-or-http2

[bagder]

The protocol is named HTTP/2 and there's no ".0" in the name or the version. I think we avoid a lot of problems by sticking to the official name.