Rework per proc and per process time usage accounting

Rework per proc and per process time usage accounting

For procs (threads) the accounting happens now lockless by curproc using
a generation counter. Callers need to use tu_enter() and tu_leave() for this.
To read the proc p_tu struct tuagg_get_proc() should be used. It ensures
that the values read is consistent.

For processes only the time of exited threads is accumulated in ps_tu and
to get the proper process time usage tuagg_get_process() needs to be called.
tuagg_get_process() will sum up all procs p_tu plus the ps_tu.

This removes another SCHED_LOCK() dependency. Adjust the code in
exit1() and exit2() to correctly account for the full run time.
For this adjust sched_exit() to do the runtime accounting like it is done
in mi_switch().

OK jca@ dlg@