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/connect.c

@@ -98,6 +98,34 @@ singleipconnect(struct connectdata *conn,
                 curl_socket_t *sock,
                 bool *connected);
 
+/*
+ * Curl_timeleft_accept() returns the amount of milliseconds left allowed for
+ * waiting server to connect. If the value is negative, the timeout time has
+ * already elapsed.
+ *
+ * The start time is stored in progress.t_acceptdata - as set with
+ * Curl_pgrsTime(..., TIMER_STARTACCEPT);
+ *
+ */
+long Curl_timeleft_accept(struct SessionHandle *data)
+{
+  long timeout_ms = DEFAULT_ACCEPT_TIMEOUT;
+  struct timeval now;
+
+  if(data->set.accepttimeout > 0)
+    timeout_ms = data->set.accepttimeout;
+
+  now = Curl_tvnow();
+
+  /* subtract elapsed time */
+  timeout_ms -= Curl_tvdiff(now, data->progress.t_acceptdata);
+  if(!timeout_ms)
+    /* avoid returning 0 as that means no timeout! */
+    return -1;
+
+  return timeout_ms;
+}
+
 /*
  * Curl_timeleft() returns the amount of milliseconds left allowed for the
  * transfer/connection. If the value is negative, the timeout time has already