Fix CPU load reporting independent of scheduler used.

Fix CPU load reporting independent of scheduler used.

- Sample CPU usage data from kern.cp_times, this makes for a far more
  accurate and scheduler independent algorithm.
- Rip out the process list scraping that is no longer required.
- Don't update CPU usage sampling on every request, but every 15s
  instead. This makes it impossible for an attacker to hide the CPU load
  by triggering 4 samplings in short succession when the system is idle.
- After reaching the steady-state, the system will always report the
  average CPU load of the last 60 sampled seconds.
- Untangling of call graph.

PR:		kern/130222
Tested by:	Julian Dunn <jdunn@aquezada.com>
		Gustau Pérez <gperez@entel.upc.edu>
		Jürgen Weiß <weiss@uni-mainz.de>
MFC after:	2 weeks

I'm unsure if some MIB standard states this must be the load average
for, eg. 300s, it looks like net-snmp isn't even bothering to implement
the CPU load reporting at all.