limit cpus when machine in use

we have a prefs option to suspend boinc calculations when the machine is in use.

On a 2-cpu machine this is overkill, it would be nice to simply free up one cpu whenever the machine is in use. This would give the box enough slack to respond to user keyboard activity and could well give full responsivity.

On a 4-cpu machine (or a twin core hyperthreaded) limiting to 3 cpus should do the trick.

Notice that as user threadfs start up, they will take precedence over BOINC because of the priority settings, the issue is around how soon windows notices mouse/keyboard activity.

Just as with the currennt client, after 3 mins (or some other user-configurable delay) the machine would revert to using the full number of cpus again.