[Oops, forgot to commit this together with things that depend on it.]
[Oops, forgot to commit this together with things that depend on it.]

First attempt at creating devfs entries for sliced devices.  Doesn't
quite work yet, so the heart of it is disabled.

Added bdev and cdev args to dsopen().

Create devfs entries in dsopen() and (unsuccessfully) attempt to make
them go away at the right times.  DEVFS is #undefed at the start so
that this shouldn't cause problems.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase