Adding Priority Scheduling feature to the subprocess

[Terry Reedy]

| Because UNIX uses priorities between +20 and -20 and Windows, via
| Process and Thread priorities, allows settings between 0 and 31, a
| uniform setting for each system should be derived.  This would be
| accomplished by giving process priority in terms of a floating-point
| value between 0.0 and 1.0 for lowest and highest possible priority,
| respectively.

I would rather that the feature use the -20 to 20 priorities and map that 
appropriately to the Windows range on Windows.