Cars are names associated to a service charge. They are stored in the
state file in a dict called "cars". Each car has its own balance
from which money can be pulled or pushed. Because of this cars and users
can't share a name.
Service charges are calculated per drive and split among all drivers
similar to fuel cost. If a driver is not known by geldschieberbot the
recipient of the "tanken" cmd is held accountable and has to collect
the service charge manually just like fuel cost.
NOTE: This change breaks geldschieberbot if the state file does not contain the field "cars" !!!
You have to add it manually !!!