Largely unify the <machine/cpu.h> headers on the m68k platforms.

Adjust the clockframe to be the same as what the common m68k interrupt
dispatch code expects.

Make the M68K_MMU_* defines real kernel options, available in opt_m68k_arch.h.
For 68030, 68040, and 68060, infer M68K_MMU_MOTOROLA. For 68010 and 68020, it
must be an explicit option (set in std.${MACHINE} or, in the case of hp300,
inferred from the specific model option).

Remove duplicated / slightly-tweaked loadustp() (load user segment table)
routines from all of the m68k ports using the shared pmap. Instead, in
pmap_init(), set up a function pointer to the appropriate mmu_load_urp*()
function in mmu_subr.s.

Provide a mechanism for a system reset controller to register to be used
to reset/halt the system.

Collect some garbage.

Add a port to the Qemu m68k "virt" platform, a 68040-based virtual machine
with VirtIO I/O devices.

This is a work-in-progress, but "successfully mounts root file system
and then complains about missing /sbin/init that is, in fact, not there"
seems like a good checkpoint.