Add set_default_subvol parameter to allow disabling the set-default step

[rmbruntz]

Some users prefer keeping subvolid=5 as the default and mounting the root via subvol=@ instead (due to GRUB issues or for organizational reasons). This adds a parameter in the config to toggle this step, as the rest of the rollback process is fully compatible with a default subvolid=5 setup.

Both the default value in the .conf and the default behavior of the script are left as true for consistency with the current version.