This fixes three resource accounting related bugs introduced by detached
threads:

 - the 'child CPU usage' fields were updated in wait4 until now - this was
   slightly buggy for a number of reasons, eg. if the exit_code writout
   faults then it's possible to trigger this code multiple times.

 - those threads that do not go through wait4 were not properly accounted.

 - sched_exit() was incorrectly assuming that current == parent. In the
   detached case p->parent is the real parent.

with this patch applied things like 'time' work again for new-style
threaded apps.