restore support for DS1820

- the DS1820 (circa 1998) is still in use in some temperature
  networks, and can be identified by the double height PR35 package,
  and a scratchpad COUNT_PER_C register value that changes during
  operation, but not by the family code in the device address,

- fixes a regression introduced in e1cdffb, resulting in sawtooth
  temperature graphs for a linear temperature change, caused by using
  a fixed COUNT_PER_C register value for DS18S20 and therefore DS1820,

- tested with both DS18S20 and DS1820 sensors.