Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Rip out the nastiness I cribbed from the EISA bus code and actually
implement the resource management code correctly, using approved
interfaces.

While I'm here, clean up a few things.

This is the rest of the MCA support; new_bus code to be exact.

Should we ever find ourselves on an RS/6000 this code should work
with few changes.