WP_Filesystem: FTP Sockets: Avoid using the file_exists() / is_exists() / exists() PemFTP functionality as it's buggy on ncFTPd servers, switching to listing the file instead which is what we use for the FTP Extension transport. Fixes #14049


git-svn-id: https://develop.svn.wordpress.org/trunk@25274 602fd350-edb4-49c9-b593-d223f7449a82