FTP: prevent the multi interface from blocking

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

tests/data/test591

@@ -63,8 +63,9 @@ TYPE I
 STOR 591
 QUIT
 </protocol>
+# CURLE_UPLOAD_FAILED = 25
 <errorcode>
-10
+25
 </errorcode>
 <upload>
 </upload>