clockintr: add clockintr_unbind()

clockintr: add clockintr_unbind()

The clockintr_unbind() function cancels any pending execution of the
given clock interrupt object's callback and severs the binding between
the object and its host CPU.  Upon return from clockintr_unbind(), the
clock interrupt object may be rebound with a call to clockintr_bind().

The optional CL_BARRIER flag tells clockintr_unbind() to block if the
clockintr's callback function is executing at the moment of the call.
This is useful when the clockintr's arg is a shared reference and the
caller needs to be certain the reference is inactive.

Now that clockintrs can be bound and unbound repeatedly, there is more
room for error.  To help catch programmer errors, clockintr_unbind()
sets cl_queue to NULL.  Calls to other API functions after a clockintr
is unbound will then fault on a NULL dereference.  clockintr_bind()
also KASSERTs that cl_queue is NULL to ensure the clockintr is not
already bound.  These checks are not perfect, but they do catch some
common errors.

With input from mpi@.

Thread: https://marc.info/?l=openbsd-tech&m=170629367121800&w=2

ok mpi@