Support for multiple register banks on SBUS devices (based on patches
from Francis Dupont).
Highlights:
romaux defines an array of register spaces.
pay attention to `vactype'.
quirks handling 4/100 idiosyncracies.
cpu.h: cpu_adjstack() won't work well in the NetBSD execve - need a
different model, stub enablertclock() for now..
exec.h: a netbsd-type exec.h file (ie. empty)
param.h: add MID_MACHINE
pmap.h: delete kernel_pmap hack - it doesn't work for us, + two proto's
vmparam.h: VM_MIN_ADDRESS = 0!