Backup Migrate 8.x not detecting private file system settings

Here's a patch that replaces variable_get for file_private_path with a call to config('system.file') and then uses method get('path.private') to retrieve the file. This only moves the problem. backup_migrate_create_destination() doesn't create the target directories, so that's probably the next thing to fix.

Edit: Also, submitted patch isn't the one I meant to post, as it introduces an unnecessary variable. I'll fix that on my next update.

Thanks for the review of work done so far.

The patch in #1 fixes the original error, but backups still do not work. This needs more changes before it's ready.