If there are already prune options configured, simply delete the maxfiles
setting. Having set both is invalid from vzdump's perspective anyways, and any
backup job on such a storage failed, meaning a user would've noticed.
If there are no prune options, translate the maxfiles value to keep-last,
except for maxfiles being zero (=unlimited), in which case we use keep-all.
If both are not set, don't set anything, so:
1. Storages don't suddenly have retention options set.
2. People relying on vzdump defaults can still use those.
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
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