[VFS-167][FTP] Add proxy support.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/vfs/trunk@1591789 13f79535-47bb-0310-9956-ffa450edef68
[VFS-167][FTP] Add comment, you need to set passive mode for most proxies.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/vfs/trunk@1591795 13f79535-47bb-0310-9956-ffa450edef68