kernel - Fix syscons cursor movement

kernel - Fix syscons cursor movement

* Recent commits broke the cursor movement test, causing
  certain cursor movements to not update the screen.

* Re-enable the asynchronous update thread, whos primary
  purpose is to fix stuttering sound and other interrupt
  issues.

* Add a sysctl kern.syscons_async which defaults to 1
  to control whether the async thread is used or not.
  This can be changed at any time.

Reported-by: marino