sysctl: Misc code cleanup

• Use defined SYS_SYSCTL names
• Use memcmp instead of explicit loop
• Use NULL instead of 0 for pointer value
• Use __FBSDID
• Reformat, improve comments in parse()

No functional changes.

Reviewed by: imp
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D26832