add CURLINFO_SCHEME, CURLINFO_PROTOCOL, and %{scheme}

[gevaerts]

Adds access to the effectively used protocol/scheme to both libcurl and
curl, both in string and numeric (CURLPROTO_*) form.

Note that the string form will be uppercase, as it is just the internal
string.

As these strings are declared internally as const, and all other strings
returned by curl_easy_getinfo() are de-facto const as well, string handling
in getinfo.c got const-ified.

[mention-bot]

@gevaerts, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bagder, @captain-caveman2k and @philipc to be potential reviewers.

[bagder]

This looks clean and I see no problems. Ready for merge!

[bagder]

thanks!