FTP: perform active connections non-blocking

1- Two new error codes are introduced.

CURLE_FTP_ACCEPT_FAILED to be set whenever ACCEPTing fails because of
FTP server connected.

CURLE_FTP_ACCEPT_TIMEOUT to be set whenever ACCEPTing timeouts.

Neither of these errors are considered fatal and control connection
remains OK because it could just be a firewall blocking server to
connect to the client.

2- One new setopt option was introduced.

CURLOPT_ACCEPTTIMEOUT_MS

It sets the maximum amount of time FTP client is going to wait for a
server to connect. Internal default accept timeout is 60 seconds.

lib/progress.h

@@ -34,6 +34,7 @@ typedef enum {
   TIMER_STARTTRANSFER,
   TIMER_POSTRANSFER,
   TIMER_STARTSINGLE,
+  TIMER_STARTACCEPT,
   TIMER_REDIRECT,
   TIMER_LAST /* must be last */
 } timerid;