streamline dpkgpm cleanup-handling

streamline dpkgpm cleanup-handling

The (unlikely) waitpid failure case should fallthrough the code just
like the other failures (and successes) instead of taking a shortcut
avoiding all the cleanup (progress) and finishing touches (log, state).

This also delays the cleanup of the progress until apt is really done
with everything and "just" has the post-invokes left to do, so the
period of 'apt looks finished as it stopped the progress' and 'apt
really finished as I have the shell-prompt back' is shorter even if
there is no progress reported anymore, so the bar lingers at 100%…

Ideally even the post-invokes would be covered by progress, but they
can have their own output and dealing with that could be hard.

Git-Dch: Ignore