. use a structure for the tag instead of an integer constant,
. add bus_space_{peek,poke}_N() (and G/C `badaddr()'),
. fix a few drivers which have dependencies on the implementation.
These changes add support for:
o The MI VMEbus framework on both MVME147 and MVME167.
o Enhancements to the existing MD bus_space(9) implementation.
o Most of the bus_dma(9) API.