Since the headers for cpu, memory and load functions are virtually the same for
all platforms, I've decided to move them into common/ dir and do some
refacotring:
* removed per-platform header files
* implemented get_cpu_count() function across all platforms. We are using it cpu
on every platform, yet not on every one this was implemented as a separate
function.
* removed platform detection through preprocessor from main: we don't need this
there anymore, since the headers are common for all platforms. CMake will
handle setting of correct source files for us now.
* Unified used defines for CPU states across all platforms and made linux use
them. Added some platform detection to cpu.h in order to set them correctly
across the platforms.
* moved getsysctl.h to common/ dir, since it's used on Net and Free BSD, and
thus become a common include.
