More cleanups:
More cleanups:

- 0 should have been -1 in previous commit (just to stay consistent),
- Spell null pointers as NULL, not 0,
- Fixed the comment about pr_usesysctl to not confuse it with boolean.

Pointed by:	bde

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase