Simplified lr_perform()'s loop to prevent busy wait

[ydroneaud]

Calling curl_multi_perform() in a loop is actually busy waiting for data being available on the socket.

The whole logic in lr_perform() can be simplified allowing more efficient processing. The function has to repeatedly do:

1) call curl_multi_perform() for curl to fetch data if needed/available 2) call check_transfer_statuses() to check download status 3) if curl needs more data, wait for the socket to have some

Once there's no need for more data, and there's no other transfer pending, lr_perform() can return TRUE.

The function can be further simplified by

• assuming curl_multi_perform() is not returning CURLM_CALL_MULTI_PERFORM require cURL 7.20, https://curl.haxx.se/libcurl/c/curl_multi_perform.html
• using curl_multi_wait() instead of select() require cURL 7.28, https://curl.haxx.se/libcurl/c/curl_multi_wait.html

This fixes https://bugzilla.redhat.com/show_bug.cgi?id=1691856

Fixes: 208edbecf3b0 ('Reduce download delays by using still_running correctly.')

[jrohel]

Thank you. Fix with nice simplification.