kernel - Fix another hard critical section issue with syscons, revamp locking

kernel - Fix another hard critical section issue with syscons, revamp locking

* Use the new mtx_spinlock() facility instead of spinlock().  The new
  facility supports reetrant locks.

* Call all sc_*() functions with the syscons lock held.

* Fix an issue in blink_screen() where a hard critical section panic
  could potentially occur.

* Fix an issue in sc_puts() where adjusting the keyboard LED could still
  cause a hard critical section panic.  We just let the keyboard LED state
  be wrong for now.