macOS: Better approximation for AvailablePhysicalMemory

On macOS, free pages does not seem to be a good indicatur for "available
physical memory". Neither inactive ones, but it answers better questions like:
"How much memory could this process use in best case scenario".

Free + Inactive aproximately matches Activity Monitor.app output.

https://developer.apple.com/library/archive/documentation/Performance/Conceptual/ManagingMemory/Articles/AboutMemory.html#//apple_ref/doc/uid/20001880-99406-TPXREF103