Add option to prefix the full subvolume path in snapshot_name

[LubosKolouch]

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[digint]

i'm not sure if this completely implements it, looks promising though.
I suspect there might be other places in the code which rely on simple $snapshot_name. not sure though, will have to look at it in detail.

[digint]

I've had closer look at it, and I fear your fix only solves part of the problem.

probably patch the path in early (directly in parse_config_line) seems the better approach to me.
Leaving the merge request open for now, as this feature still seems very useful to me.