configurations and make it opt-in for those who want it. LINT will
still build it.
While it may be a perfect win in some scenarios, it still troubles users
(see PRs) in general cases. In addition we are still allocating resources
even if disabled by sysctl and still leak arp/nd6 entries in case of
interface destruction.
Discussed with: qingli (2010-11-24, just never executed)
Discussed with: juli (OCTEON1)
PR: kern/148018, kern/155604, kern/144917, kern/146792
MFC after: 2 weeks
c41eae2d13