DEBORPHAN/10-whitelist: rework fix for broken keep file handling of deborphan

An empty file doesn't help either, so we need at least a file
with a newline. Given that we're already touching the file
based on its hard-coded filename, let's just append the package
in question and avoid invoking deborphan at all. Especially
as deborphan returns with failure when a package is already
listed:

| deborphan: dnsutils: duplicate entry.

We need to handle this anyway, if grml-live is re-executed on
an existing system (which isn't unusual).