trigger a breakpoint with this chip.
- Fiddle the right bits in the cn input and output routines to disable port
interrupts and enable visibility of the masked interrupt status bits.
- Register a shutdown final event handler to put the chip back in the mode
that the prom expects.
- Get the initial mode from the prom settings and don't clobber the mode
on open.
- Copy output into an internal ring buffer instead of accessing the tty
outq directly in the interrupt handler. This fixes a problem where
garbage would show up in the output stream.
- Reset the console port completely and reprogram all the parameters
before enabling it. This fixes seemingly random hangs on startup
when using a fast interrupt handler.
- Add minimal locking in place of spls.
- Remove dead code and minor cleanups.
