cpu plugin: Various, random fixes and improvements:

* Add Pierre-Yves to the list of copyright holders.
* Make sure the entire file is consistently indented.
* Renamed the "ReportActive" config option to "ReportByState"
  (with negated logic).
* Renamed internal functions to better reflect their use.
* Let cpu_stage() (née submit()) work on individual counters / derives
  rather than an array, removing logic from the cpu_read() function.
* Rename CPU_SUBMIT_* to CPU_STATE_*.
* Move legacy behavior it its own function, cpu_commit_without_aggregation().
Merge remote-tracking branch 'github/pr/734'