Raise WARNS to 6 and generally clean up:

Raise WARNS to 6 and generally clean up:
- Constify and use size_t instead of int;
- Merge and add variables as appropriate;
- Use more appropriate printf() format specifier, and replace
  printf() with fwrite() in one instance;
- Replace name2oid() with sysctlnametomib().  Retain old error
  message when the sysctl's name cannot be resolved to a MIB.

Based-on-coincident-patches-by: myself, and Peter Schuller
                        <peter_dot_schuller_at_infidyne_dot_com>
Refined-with-input-from: joerg
OK'ed-by: joerg