Support the intel_pstate scaling driver

I noticed that the CpuFreqDef config option was only partially implemented. The value was parsed, but the never used. So I took the liberty of re-purposing it to mean sort of the opposite, namely the frequency governor to use when running a job step in case the job doesn't explicitly provide any --cpu-freq option.

I also changed the default of the CpuFreqGovernors option to be "ondemand,performance", since ondemand isn't available with the intel_pstate driver.

Otherwise the patch should be relatively straightforward and only changes a few minor things here and there.