CLEANUP: cfgparse: modify preprocessor guards around numa detection code

CLEANUP: cfgparse: modify preprocessor guards around numa detection code

numa_detect_topology() is always define now if USE_CPU_AFFINITY is
activated. For the moment, only on Linux an actual implementation is
provided. For other platforms, it always return 0.

This change has been made to easily add implementation of NUMA detection
for other platforms. The phrasing of the documentation has also been
edited to removed the mention of Linux-only on numa-cpu-mapping
configuration option.