Fix detaching of uart(4) devices.
Fix detaching of uart(4) devices.

With our new TTY layer we use a two step device destruction procedure.
The TTY first gets abandoned by the device driver. When the TTY layer
notices all threads have left the TTY layer, it deallocates the TTY.

This means that the device unit number should not be reused before a
callback from the TTY layer to the device driver has been made. newbus
doesn't seem to support this concept (yet), so right now just add a
destructor with a big comment in it. It's not ideal, but at least it's
better than panicing.

Reported by:	rnoland

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