md(4): add support for unmapped preloaded images -- imp hacks

[imp]

Summary

Making copies directly on physical addresses eliminates the need to
remap the images into KVA, and should be equally fast on DMAP arches.

XXX HAS THE HACKS

[imp]

This is just https://reviews.freebsd.org/D51128, lightly hacked to make it compile with what I think we might need. It seems to work, but my test jig didn't let me access the md0 disks since it just is a test booting jig. Things showed up in the dmesg correctly (and if I booted the right kernel, didn't panic anything).

[imp]

And to be clear, kib wrote this all, I just adopted the last chunk in a way I think might be right????

[kib]

The update seems to be correct, I did exactly the same in D51128

[imp]

In D51466#1175244, @kib wrote:

The update seems to be correct, I did exactly the same in D51128

OK. I had to tweak things after git arc patch...

Do you want to commit this then? Or would you rather I do it?

[imp]

kib's version is better