nspawn: make use of the RequestStop logic of scope units

nspawn: make use of the RequestStop logic of scope units

Since time began, scope units had a concept of "Controllers", a bus peer
that would be notified when somebody requested a unit to stop. None of
our code used that facility so far, let's change that.

This way, nspawn can print a nice message when somebody invokes
"systemctl stop" on the container's scope unit, and then react with the
right action to shut it down.