Completely rewritten scsipi_xfer execution engine:
- All xfers are queued in the mid-layer, rather than doing so in an
  ad-hoc fashion in individual adapter drivers.
- Adapter/channel resource management in the mid-layer, avoids even trying
  to start running an xfer if the adapter/channel doesn't have the resources.
- Better communication between the mid-layer and the adapters.
- Asynchronous event notification mechanism from adapter to mid-layer and
  peripherals.
- Better peripheral queue management: freeze/thaw, sorted requeueing during
  recovery, etc.
- Clean separation of peripherals, adapters, and adapter channels (no more
  scsipi_link).
- Kernel thread for each scsipi_channel makes error recovery much easier
  (no more dealing with interrupt context when recovering from an error).
- Mid-layer support for tagged queueing: commands can have the tag type
  set explicitly, tag IDs are allocated in the mid-layer (thus eliminating
  the need to use buggy tag ID allocation schemes in many adapter drivers).

There is a lot more work to do, but this correctly functions for the most
part on several file servers I run.
Adapt to scsipi API changes.
Adapt to scsipi API changes, and add Tagged Queueing support.
Adapt to scsipi API changes.
Adapt for scsipi API changes.
Adapt to scsipi API changes.
Add the SCSI ethernet device.
Adapt to scsipi API changes.
Adapt to scsipi API changes.
Ooops, forgot to initialize a variable.
Don't cast return value of config_found() to void, as it's not consistant with
the rest of the driver.
Add an asynchronous event for the adapter to notify the midlayer that
xfer mode parameters have changed for an I_T Nexus.
Use the ASYNC_EVENT_XFER_MODE callback.
Let the midlayer tell if we can use tagged queueing, and what sort of
tag to use.
Adapt to scsipi API changes.
Don't sleep waiting for a CCB; if we get to the CCB allocation phase, we're
guaranteed to have one.
Brain'o.
Don't sleep waiting for a CCB; if we get to the CCB allocation phase, we're
guaranteed to have one.
- Sync with -current (untested as yet).
- One cosmetic change.
Fix compilation error when ATAPI_DEBUG_PROBE is defined.