Current and limit temperature read from system files instead of from vcgencmd utility.

- The utility vcgencmd is not preinstalled in all distributions, especially not
  in minimalistic ones. It is recommended to read temperatures from system files:
  - Current temp: /sys/class/thermal/thermal_zone0/temp
  - Limit temp: /sys/class/thermal/thermal_zone0/trip_point_0_temp
- The reading of the limit temperature has been included for future use. Is is
  the maximal allowed temperature for Raspberry Pi. Usually, at reaching this
  value the system is forced to shutdown.
- The naming of temperature variables has been unified.