Note that sys/vm can cause double-free panics too if invariants are enabled

This documents PR # 192098 to some degree

Sponsored-by: EMC / Isilon Storage Division
Only call the callbacks once (forgot that sysctl(3) calls the KPI twice -- once for get, the other for set)
Start test driver for memguard

The goals for testing are covered in ../README

Sponsored-by: EMC / Isilon Storage Division