'RELENG_5_3_0_RELEASE'.

This commit was manufactured to restore the state of the 5.3-RELEASE image.

happens because the sio device was never opened and com->tp is
therefore NULL.  ttygone can't swallow a NULL, so guard against that
possibility.  Other places in this function make similar checks, so I
believe this is correct.

future:

rename ttyopen() -> tty_open() and ttyclose() -> tty_close().

We need the ttyopen() and ttyclose() for the new generic cdevsw
functions for tty devices in order to have consistent naming.

Move dtrwait logic to the generic TTY level.

it's in the way even more. Basicly: remove all alpha specific console
support from gfb(4), sio(4) and syscons(4). Rewrite the alpha console
initialization to be identical to all other platforms. In a nutshell:
call cninit().
The platform specific code now only sets or clears RB_SERIAL and thus
automaticly causes the right console to be selected.

sio.c:
o  Replace the remote GDB hacks and use the GDB debug port interface
   instead.
o  Make debugging code conditional upon KDB instead of DDB.
o  Call kdb_alt_break() instead of db_alt_break().
o  Call kdb_enter() instead of breakpoint().
o  Remove the ugly compatibility of using the console as the debug
   port.

Change the return type for t_break to void.

Add t_ioctl (more about this later).

  'tty:' prefix is largely redundant.
- Fix the priority of the low-priority TTY SWIs that are hung off of the
  softclock thread.

Submitted by:	bde (2)

copies.

No current line disciplines have a dynamically changing hotchar, and
expecting to receive anything sensible during a change in ldisc is
insane so no locking of the hotchar field is necessary.