Restore Copyright Years in src/*.c src/*.h

Restore Copyright Years in src/*.c src/*.h

- Restore the Copyright Years, so that rrdtool-release will update the
  years in the future again as intended.
  The years are not present any more since the following commit:
  https://github.com/oetiker/rrdtool-1.x/commit/1964f6f
- rrdtool-release expects a 4 digit year for the substitution:
  s/Copyright.+?Oetiker.+\d{4}