curl_multi_fdset: correct fdset with FTP PORT use

After a PORT has been issued, and the multi handle would switch to the
CURLM_STATE_DO_MORE state (which is unique for FTP), libcurl would
return the wrong fdset to wait for when curl_multi_fdset() is
called. The code would blindly assume that it was waiting for a connect
of the second connection, while that isn't true immediately after the
PORT command.

Also, the function multi.c:domore_getsock() was highly FTP-centric and
therefore ugly to keep in protocol-agnostic code. I solved this problem
by introducing a new function pointer in the Curl_handler struct called
domore_getsock() which is only called during the DOMORE state for
protocols that set that pointer.

The new ftp.c:ftp_domore_getsock() function now returns fdset info about
the control connection's command/response handling while such a state is
in use, and goes over to waiting for a writable second connection first
once the commands are done.

The original problem could be seen by running test 525 and checking the
time stamps in the FTP server log. I can verify that this fix at least
fixes this problem.

Bug: http://curl.haxx.se/mail/lib-2011-10/0250.html
Reported by: Gokhan Sengun

lib/multi.c

@@ -816,20 +816,12 @@ static int waitconnect_getsock(struct connectdata *conn,
 }
 
 static int domore_getsock(struct connectdata *conn,
-                          curl_socket_t *sock,
+                          curl_socket_t *socks,
                           int numsocks)
 {
-  if(!numsocks)
-    return GETSOCK_BLANK;
-
-  /* When in DO_MORE state, we could be either waiting for us
-     to connect to a remote site, or we could wait for that site
-     to connect to us. It makes a difference in the way: if we
-     connect to the site we wait for the socket to become writable, if
-     the site connects to us we wait for it to become readable */
-  sock[0] = conn->sock[SECONDARYSOCKET];
-
-  return GETSOCK_WRITESOCK(0);
+  if(conn && conn->handler->domore_getsock)
+    return conn->handler->domore_getsock(conn, socks, numsocks);
+  return GETSOCK_BLANK;
 }
 
 /* returns bitmapped flags for this handle and its sockets */