We're mostly saying just "curl" in lower case these days so here's a big
cleanup to adapt to this reality. A few instances are left as the
project could still formally be considered called cURL.
1) Renamed curl_tlsinfo to curl_tlssessioninfo as discussed on the
mailing list.
2) Renamed curl_ssl_backend to curl_sslbackend so it doesn't follow our
function naming convention.
3) Updated sessioninfo.c example accordingly.
