Add mvkGetRegistryID() function to check OS support for registry ID.
Rename build setting MVK_FORCE_LOW_POWER_GPU to MVK_CONFIG_FORCE_LOW_POWER_GPU
and make it an env var / build setting combo.
Rename env var MVK_LOG_LEVEL to MVK_CONFIG_LOG_LEVEL.
Rename MVKLogging.mm to MVKLogging.cpp.