Move the default file list to a separate file in /etc/sysmocom/backup.d,
which is itself included in the backup. Also create a symlink to the
most recent backup.
Creating a new file in /etc/sysmocom/backup.d/ that ends with .backup
will be read during the backup and then be restored. This way we can
easily extend the files to backup.
* when restoring links e.g. for systemd files
the links will be created with content// prefix
* use transform instead of strip to remove
the "content/" prefix
* bump PR
Additional applications might store files in /etc/default and it
is the easiest to backup up all of them. This has the danger that
after an upgrade the default file is not compatible/optimal but
we will need to deal with that once it occurs.