This change allows fan outputs to automatically turn on/off when the
associated nozzle temperature of an extruder is above/below a threshold
temperature.
Multiple extruders can be assigned to the same pin in which case the fan
will turn on when any selected extruder is above the threshold.
It also makes the M42 command compatible with the M106/M107 command.
The majority of the logic in this change will be evaluated by the
compiler at build time (i.e, low code space requirements).
Robert F-C