fix #3307: make it possible to set protection for backups

fix #3307: make it possible to set protection for backups

A protected backup is not removed by free_image and ignored when
pruning.

The protection_file_path function is introduced in Storage.pm, so that
it can also be used by vzdump itself and in archive_remove.

For pruning, renamed backups already behaved similiar to how protected
backups will, but there are a few reasons to not just use that for
implementing the new feature:
1. It wouldn't protect against removal.
2. It would make it necessary to rename notes and log files too.
3. It wouldn't naturally extend to other volumes if that's needed.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>