optionally use the full subvolume path as default snapshot_name

Currently btrbk uses only the subvolume (base)name as a prefix for naming snapshots & backups. The information where a subvolume comes from/goes in the larger fs structure is lost.
This might be a problem in naming schemes where the actual subvolume has rather generic names, it's definitely a problem when you have two subvolumes of the same name, e.g. `$REALROOT/@home/user1/Documents`, `$REALROOT/@home/user2/Documents`, and so on.

It's possible to set snapshot_name for each affected subvolume, of course, but that's quite fiddly and, considering it has to be set on both ends in a split snapshot-only/pull-only setup, error prone.

How about a config file switch that changes the default snapshot_name to a sensibly escaped version of `subvolume-name`, the path given in the `subvolume` section? Something like `@home-user1-Documents` in the above case?

Start by tracing configuration handling and where the default snapshot_name is selected from the subvolume path, including snapshot-only and pull-only flows. Done means an optional setting produces consistent, safely escaped full-path-based names while preserving the existing default behavior.

Written by the indexing model from the issue text.