Merge PR 'Curl improvements'

Curl versions older than 7.57.0 (found on Debian Stretch and older) do
not implicitly initialize openssl, which leads https requests to fail.

In the course of debugging this issue verbose curl requests were needed,
so that has also been added as a cvar.

https://github.com/DarkPlacesEngine/darkplaces/pull/36

commits: ceaa16c78a1f43165206566d4f9c872664f7e680
1a711173535d4b5f4364b66177745aab165330d8

Signed-off-by: bones_was_here <bones_was_here@xa.org.au>