Anatol Pomozov authored Mar 09, 2020 and Allan McRae committed May 09, 2020

To be able to run multiple download in parallel efficiently we need to
use curl_multi interface [1]. It introduces a set of APIs over new type
of handler 'CURLM'.

Create CURLM object at the application start and set it to global ALPM
context.

The 'single-download' CURL handle moves to payload struct. A new CURL
handle is created for each payload with intention to be processed by CURLM.

Note that curl_download_internal() is not ported to CURLM interface due
to the fact that the function will go away soon.

[1] https://curl.haxx.se/libcurl/c/libcurl-multi.html



Signed-off-by: Allan McRae <allan@archlinux.org>