[counters] Add system counters

After discussion with Kumpera, it was decided to load them lazily when calling mono_counters_foreach for the first time. It has to be inside the mono runtime, and not in a profiler because it accesses proclib API which is all MONO_INTERNAL functions.
[counters] Edit get process pid and /proc/loadavg according to Paolo review
[counters] Edit according to Kumpera review
Merge pull request #1020 from ludovic-henry/pr7-system-counters

[counters] Add system counters