Move the CLST_PENDING check and TAILQ_REMOVE() in
clockintr_schedule_locked() into a dedicated function,
clockintr_cancel_locked(). clockintr_schedule_locked() no longer
implicitly cancels a pending clockintr: it is the caller's
responsibility to check for CLST_PENDING and cancel any pending
expiration before calling clockintr_schedule_locked(). We can skip
the CLST_PENDING check during the dispatch loop because we know for
certain the clockintr in question is pending.

This is more verbose but I think it is less surprising. Both
functions do one thing.

No intentional functional change.

ok miod@ tb@

While here, fix one wrong parameter type and three inconsistent
parameter names in the SYNOPSIS.