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.c

@@ -169,6 +169,10 @@ void Curl_pgrsTime(struct SessionHandle *data, timerid timer)
     data->progress.t_startsingle = now;
     break;
 
+  case TIMER_STARTACCEPT:
+    data->progress.t_acceptdata = Curl_tvnow();
+    break;
+
   case TIMER_NAMELOOKUP:
     data->progress.t_nslookup =
       Curl_tvdiff_secs(now, data->progress.t_startsingle);