Darwin fixes

[dkechag]

Various fixes for MacOS to make it behave similar to the Linux platform:

• cpu will now return the actual CPU model name (e.g. Intel i7 / Apple M1 etc) and cpu_type will return the architecture (x86_64, arm64). I think the confusion started because of the cpu_type selected as the hash key to store architecture is very similar to the "CPU Type" Apple profiler entry which was the CPU model. I don't think the actual computer model (MacBook Pro etc) should be returned in any "cpu" field. It would be fine for a "machine_model" field or something like that.
• cpu_cores now returns the logical core count in the recommended way (hw.logicalcpu), to match the Linux behaviour. I added a physical_cores field which returns the physical count that was reported previously as cpu_cores. Similar functionality can be added to Linux.
• POD clarifies behaviour and tests are added.

If some of the changes are not desired, I could take them out of this PR of course.

[Tux]

Add to ChangeLog, version++'d and made style a bit more consistent. Pushed. Thanks!