Fix CurlFile::Exists/Stat for check ftp directory with proper cwd method.

using NOCWD for checking ftp directory exists is less standard, and will just return empty list (no error) for non-existed remote directory, e.g. on xbmc's ftp server.

try this command line to known what I said:
curl -v ftp://user:pwd@ftpserveraddr/non_existed_dir/ --ftp-method nocwd

This PR fixed this issue by using singlecwd mode of curl for checking directory exists/stat.