MPSAFE TTY - Fix deadlock in reporting of probe errors.

 MPSAFE TTY - Fix deadlock in reporting of probe errors.

* When sio can't drain it reports the fact, but it was doing it while
  still holding com_lock.  Rearranging the lock fixes the problem.

* Clean up some unnecessary tty_tokens in critical code paths and rearrange
  code to not hold com_lock across tty_token acquisition calls.

* Release the com_lock around breakpoint() (<enter>~^B sequence) to
  avoid a deadlock.

* Detect and ignore a reentrant kprintf() to try to avoid a deadlock.
  The detection flag is also reset by a panic.