Heads up: current needs testing

Hey folks,

after running in some uses of unitialized OF handles in the sparc64
MD code I added a few KASSERT that should catch those.

I'll test kernels on various hardware, but I have only a limited set
of machines.

Typical failure mode: during autoconfig (so before mounting root) the
kernel will fail with one of the new KASSERT. This means it is safe
to just copy a new test kernel to, say, /netbsd.test and then reoot
with that kernel:

	shutdown -b netbsd.test -r now

If it fails (panic), please report the output of the ddb "bt" command.
This test method should not put your existing install at risc.

Thanks,

Martin