* Select dense or sparse space based on the prefetchable flag, like the kernel
does. In particular, non-prefetchable mappings are forced to sparse space.
* If no window is matched, return MAP_FAILED, rather than falling through and
blowing chunks.
* Ignore addr_shift (set it to 0) for dense mappings.
in the alpha sysarch(2). In a perfect world, these would go away in
favor of a generic bus space mapping framework, but XFree86 is a long
ways away from being able to use one.
while it's sort of nice, it's sufficiently hard to do this properly
for the general case that, in the long run, it probably won't make sense
to support in the way i was planning to. (There are serious problems
with doing this, on multiprocessors.)
