Deal with different precisions in 1w temperatures

DS1822 and DS18B20 provide an additional decimal digit of precision,
which used to be discarded. This change introduces a ow_temp_t type to
store the number of decimal digits at no additional memory usage.
Since temperature is no longer 16 but 14bit (13Bit + sign) six characters will be enougth.
2^13 =8192 = 4 characters  + sign + '0' = 6 characters.