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/test592

@@ -52,6 +52,7 @@ Moooooooooooo for 592
 s/^PORT (.*)/PORT/
 s/^EPRT \|1\|(.*)/EPRT \|1\|/
 </strippart>
+# a 421 response must prevent further commands from being sent
 <protocol>
 USER anonymous
 PASS ftp@example.com
@@ -61,10 +62,10 @@ EPRT |1|
 PORT
 TYPE I
 STOR 592
-QUIT
 </protocol>
+# 28 == CURLE_OPERATION_TIMEDOUT
 <errorcode>
-10
+28
 </errorcode>
 <upload>
 </upload>