curl: Check completion in curl_multi_do()

curl: Check completion in curl_multi_do()

While it is more likely that transfers complete after some file
descriptor has data ready to read, we probably should not rely on it.
Better be safe than sorry and call curl_multi_check_completion() in
curl_multi_do(), too, just like it is done in curl_multi_read().

With this change, curl_multi_do() and curl_multi_read() are actually the
same, so drop curl_multi_read() and use curl_multi_do() as the sole FD
handler.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190910124136.10565-4-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit 948403bcb1c7e71dcbe8ab8479cf3934a0efcbb5)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>