space handles to describe the task file registers, aux register, 16 bit
data register and 32 bit data register.
The wdc softc structure has been moved to a separate file.
Added support for multiple attachments by separating the core of the
probe and attach functions from the mainbus probe and attach routines.
Added a new wdc flags 32_BIT so that the wd can determine 32 bit xfer
support from the wdc device.
