As pointed out in Bug report #3579064, curl_multi_perform() would
wrongly use a blocking mechanism internally for some commands which
could lead to for example a very long block if the LIST response never
showed.
The solution was to make sure to properly continue to use the multi
interface non-blocking state machine.
The new test 1501 verifies the fix.
Bug: http://curl.haxx.se/bug/view.cgi?id=3579064
Reported by: Guido Berhoerster
