Properly account for prefetchable memory when a request is being made.

Properly account for prefetchable memory when a request is being made.
We allow the request to go through if it matches either a prefetchable
or a non-prefetchable part of the bridge.  We do not check to make
sure it is the right kind of memory because most drivers to not yet
properly set RF_PREFETCHABLE (only cardbus seems to do so, and I'm not
entirely sure it does it right).  RF_PREFETCHABLE was invented for
cardbus, so hasn't been properly documented yet.

This is still overridable by hw.pci.allow_unsupported_io_ranges, but
the need for that is greatly reduced, especially for the nvida driver.

Approved by: re
Reviewed by: jhb and many testers
Submitted by: Matt Emmerton (although this has been reworked somewhat)