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

lib/pingpong.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2012, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -424,6 +424,9 @@ CURLcode Curl_pp_readresp(curl_socket_t sockfd,
            it may actually contain another end of response already! */
         clipamount = gotbytes - i;
         restart = TRUE;
+        DEBUGF(infof(data, "Curl_pp_readresp_ %d bytes of trailing "
+                     "server response left\n",
+                     (int)clipamount));
       }
       else if(keepon) {