make unit of all temperatures selectable

make unit of all temperatures selectable

This introduces a new configuration variable called "temperature_unit",
specifying the unit of all temperature sensors. To achieve this, each
object outputting a temperature has to call temp_print() like so:
| temp_print(p, p_max_size, <temp val as double>, <unit of val>);
to specify the input temperature unit, either one of the constants
TEMP_CELSIUS or TEMP_FAHRENHEIT.