functions (which only work on memory and i/o space) to i386_memio_*,
and make bus_space_* in bus.h be #defines which invoke them. This makes
life easier for people who need to define the all of the bus_space functions
so that they work on spaces other than memory and I/O space. Also, add
an _i386_memio_map function which is like i386_memio_map but doesn't do
the extent map checking or allocation. _i386_memio_map and i386_memio_*
are for use only by machine-dependent code.
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