Utility to get and set the kernel state at runtime, features:

 * get or set the system state at runtime,
 * handle an object up to CTL_MAXNAME levels,
 * print the right object also with an empty level name,
 * options to show the properties of a parameter,
 * output explicitly indicated by the options,
 * options not mutually exclusive,
 * output via libxo in human and machine readable formats,
 * isolated code to manage opaque values,
 * some string value is parsed to show structured output,
 * avoid non-primitive data types hardcode via libnv,
 * debug without recompiling the kernel with SYSCTL_DEBUG.