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().
memory: adjust safeguard to changes from previous commit
memory: make linux slab reporting work with older kernels
Merge remote-tracking branch 'github/pr/734'
Merge remote-tracking branch 'github/pr/697'
src/Makefile.am: Make sure plugins only export "module_register".

While there, put the LDFLAGS shared between all plugins into a separate
variable.