Apply patch in PR port-alpha/48697 (with some tweaks).

Tru64 UNIX's disklabel is the same format as BSD disklabel,
and it seems Tru64 stores incorrect geometry values in
d_nsectors (sectors/track) and d_secpercyl (sectors/cylinder).
d_secperunit seems always reliable so use it to get
dlsec (sectors/track) and dlcylsize (sectors/cylinder) values.

Should be pulled up to netbsd-6 branches.
sprinkle __unused()
more __unused()
Remove !BSD and lint branches.
Remove majority of KVM usage.
Make missing kern.evcnt an error, don't try to fallback to kmem use.
Add test environment considerations from tests.atf.7.

Bump date in an attempt to avoid provoking wizd(1).
Drop kvm-based access for driver statistics.
Remove remaining kvm glue.
GC unused variables.
Call ifp->if_output in revarprequest with KERNEL_LOCK held

Otherwise, it hits KASSERT(KERNEL_LOCKED_P()) in ether_output
when nfs_boot fails and tries RARP.