cpu plugin: Various, random fixes and improvements:

* Add Pierre-Yves (@pyr) 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()`.

@pyr this is changing quite a bit of the code you wrote early this year, so I'd appreciate your opinion on this. Sorry this is such a mixed bag of changes, but it's kind of hard to pull apart in retrospect :(