Add "intermediate" folder support.

[eoehen]

Specifically with the gzip feature, we'd love to have an "intermediate" folder (i.e. /tmp) for creating the backups. Background is, that we actually mounted a remote-fs as /backup (sshfs or one could think of sftp or Amazon S3 or CIFS mounts). In those cases you would only want to write the packaged files there

[eoehen]

Extracted from Various feature requests created by @pfeigl.