Subset of power patch: subtractive decoding

I've teased out the subtractive bridge decoding patches from the
larger power patches.  Please test them and let me know how things
go.  This eliminates hw.pci.allow_unsupported_io_range by fixing the
underlying problem this was kludging around.  Please test it and let
me know how it is working for you.  It should only make things better,
and make nothing worse.

This won't fix the other problems that the more complete power patches
attempt to fix.

http://people.freebsd.org/~imp/pci-subtractive.diff

Warner