__HAVE_NEW_PMAP_68K glue.

Switch to common m68k vector table.

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.

Add a driver for the Qemu Virtual System Controller that can be used
to halt or reset the system.

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.