enhance sysctl kern.expose_address

[christos%zoulas.com@localhost (Christos Zoulas)]

This patch:

https://www.netbsd.org/~christos/kmem_expose_address.diff

Changes kern.expose_address to a tri-state variable:

0: no access
1: access only to processes that have opened /dev/kmem
2: access to every process

It also optimizes the access control to happen only once per sysctl (not
once per process), and disallows it for procfs access.

I am planning to commit this soon, so please let me know if you see any
issues.

christos