Added a frequency error calibration mode to rtl_fm

The rtl_fm utility may now be used to calibrate the frequency error of a
DVB-T dongle using the "-c" command line switch:

```
  [-c] Do frequency error calibration
      Frequency error calibration will only work for FM.
      Use a higher sample rate, like -s170k, to handle
      large frequency errors. A strong and noise free
      signal is needed for good results. For example, use a
      broadcast FM station for calibration. Let the
      calibration run for some minutes until the values
      have stabilized. Rerun with -p option using the
      suggested ppm_error. Try this on a couple of stations.
      Repeat until the error is stable at about zero.
      Example: rtl_fm -f99.3M -s170k -r48k -c - | aplay -r48k -fS16_LE -
```