clockintr: support an arbitrary callback function argument

clockintr: support an arbitrary callback function argument

Callers can now provide an argument pointer to clockintr_establish().
The pointer is kept in a new struct clockintr member, cl_arg.  The
pointer is passed as the third parameter to clockintr.cl_func when it
is executed during clockintr_dispatch().  Like the callback function,
the callback argument is immutable after the clockintr is established.

At present, nothing uses this.  All current clockintr_establish()
callers pass a NULL arg pointer.  However, I am confident that dt(4)'s
profile provider will need this in the near future.

Requested by dlg@ back in March.