curl-util: fix downloads from file:// URLs

curl-util: fix downloads from file:// URLs

if we try to open file:// URLs that don't exist, we'll not get IO/timer
events about it, hence it is not sufficient to check for completion in
these events. Let's add a defer event, to deal with that.

Also, curl_multi_info_read() is a queue, make sure to handle all events
that might be queued.