kernel - revamp mtx_spinlock()

kernel - revamp mtx_spinlock()

* Revamp mtx_spinlock(), add mtx_spinlock_try(), and add mtx_spinunlock().

* Enter a proper hard critical section when using mtx_spinlock*(),
  just like the normal spinlock() code does.

  The difference is that mtx spinlocks have a ref count and thus are
  reentrant.

* mtx_spinlock*() is not used in the code yet.  A followup commit will
  begin using it for the syscons lock.