Cleanup some of the newbus infrastructure.

Cleanup some of the newbus infrastructure.

* Change the device_identify API to return success/failure, like
  most of the other newbus methods.  This may be used for conflict
  resolution in the future.

* Clearly document the device_identify method and formalize its use
  by adding discrimination between initial bus probes and bus rescans.
  Do not re-execute static identification code that has already been run
  every time a new driver is added at run-time.

* Clearly document the do-ISA-last hack.

* Provide generic routines for the most common device_identify operations
  (psueo or synthesized devices that operate under other devices, such as
  lpt operating under ppbus, which are not 'scanned' by the parent bus).

* Remove the hacks that install and initialize the nexus device.  Instead,
  use the existing DRIVER_MODULE infrastructure to install nexus under
  root_bus.

* Document the boot-time initialization path so it doesn't take the next
  guy 8 hours to figure out what code is actually being run when.