sched/fair: add tunable to set initial task load

The choice of initial task load upon fork has a large influence
on CPU and OPP selection when scheduler-driven DVFS is in use.
Make this tuneable by adding a new sysctl "sched_initial_task_util".

If the sched governor is not used, the default remains at SCHED_LOAD_SCALE
Otherwise, the value from the sysctl is used. This defaults to 0.

Signed-off-by: "Todd Kjos <tkjos@google.com&gt;">