Modularize the snapshotting and backup drivers

Move the various backup backends (and their configuration) into separate loadable modules. Also make the snapshotting itself a module, to be able to support other snapshotting technologies (e.g. ZFS snapshots). This will also require some rearrangements of configuration variables, as they should be grouped by the plugin that requires it. Plugins and configuration sections should be prepended with a string, e.g. "snapshot-lvm", "snapshot-zfs" or "backup-tar", "backup-rsync" or "backup-rsnap". The same naming scheme should be used for naming the plugins, e.g. "backup-tar.pm" or "snapshot-zfs.pm". The main configuration section will then provide options to choose the desired snapshotting or backup module, e.g. "snapshot=zfs", "backup=rsync"