* 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.