Provided Basic Support for -l N option on windows platform.

I have implemented basic support for **-l N** on windows platform.

Tested with platform==msvc and platform==mingw.

Values returned from OS are filtered with basic filter, to minimize load variation caused by short time between consecutive calls.

I have tested it on 2 core machine with HT and 4 core one, and CPU load in Task Manager matches one provided in -l option.

Any suggestion and improvements are moe than welcome.


If I get documentation right, `GetSystemTimes` would return false then, so on XP without SP1 it would same as before my my changes - returning `-0.0`.

I did not find any simple way to provide workaround XP without SP1 - and I think that with XPs end of life, and Service Packs being available it is not worth the effort.