save and restore selection states before/after calling dpkg

save and restore selection states before/after calling dpkg

dpkg decides certain things on its own based on selections and
especially if we want to call --pending on purge/remove actions, we need
to ensure a clean slate or otherwise we surprise the user by removing
packages we weren't allowed to remove by the user in this run (the
selection might be an overarching plan for the not-yet "future").

Ideally dpkg would have some kind of temporal selection interface for
this case, but it hasn't, so we make it temporal with the risk of
loosing state if we don't manage to restore them.