Until now download queues entries were created for each file, but only
removed in the unlikely event of changed download server. This leaked
about 4200 byte per file. Fix this by:
- Create a new function CL_RemoveFromQueue() that removes a queue entry
and call it everytime when a download either finished, failed or
aborted.
- Retire the 'download done' state, just the queue entries associated
with a download handle to NULL to communicate that the handle is
unused.
- Cleanup the full queue at shutdown.
Loading libcurl at runtime instead of linking it at compile time makes
things a lot easier and more reliable on Windows. On other platform
libcurl can be installed as optional dependency instead as an hard one.
