Backend should use subfolders when available

[Diego Peinador]

Some backends can't handle many files in a single folder (i.e. FTP) and when this limit is reached Duplicity can't check existing backups and all new backups are full (even when Duplicity is configured to create incremental backups if last full isn't so old).

This limitation in the backend can be bypassed if duplicity could use subfolders to store its data. For instance one folder with the manifest files and then volumes for each backup in their own folders.

In some ftp backends this couldn't be enough, and a bigger default volume size may come in handy.