[PATCHv2] Improve use of select in http backend

I've split my previous patch into two, since the two parts actually do
different things.

The first patch uses curl_multi_fdset to use the file descriptors of the http
connections in the call to select, in effect waking up immediately when new
data arrives.

The second patch uses curl_multi_timeout (if available) to get a better
recommendation for how long to sleep from curl instead of always sleeping
for 50ms.