This takes a Dbus iterator which has been entered into a
dictionary and prints out each key and value. It requires
a mapping which maps keys to types and units. For simple
cases the mapping will consist of a dbus type character
and a units string, e.g. dBm, Kbit/s etc. For more complex
printing which requires processing the value the 'units'
void* cant be set to a function which can be custom written
to handle the value.