use dpkg --unpack --recursive to avoid long cmdlines

Having long commandlines split into two is a huge problem if it happens
and additionally if we want to introduce planners which perform less
micromanagment its a good idea to leave the details for dpkg to decide.

In practice this doesn't work yet unconditionally as a bug is hiding in
the ordering code of dpkg, but it works if apt imposes its ordering so
this commit allows for now at least to solve the first problem.