core: rework how we count the n_on_console counter

core: rework how we count the n_on_console counter

Let's add a per-unit boolean that tells us whether our unit is currently
counted or not. This way it's unlikely we get out of sync again and
things are generally more robust.

This also allows us to remove the counting logic specific to service
units (which was in fact mostly a copy from the generic implementation),
in favour of fully generic code.

Replaces: #7824