Define VM-object flags and pmap_enter() flags allowing the general-purpose
VM subsystem to request tag-free mappings for pages associated with
suitably flagged objects.  For now, set those flags on vnode objects, as
our filesystems can't maintain tags persistently -- and, perhaps for some
time to come, they probably shouldn't do given our current address-space
model (although there is some neato stuff to do here once we decide to
break that assumption).

For now, no pmap-layer changes to implement this as we're possibly
encountering a problem with exception priorities, and there's also no
suitable fault handler for this case (it should map to SIGPROT or SIGSEGV
but doesn't yet).