lapic timer: Improve lapic timer vector code

lapic timer: Improve lapic timer vector code

- Check for a non-zero td->td_nest_count before allowing the processing
  to occur.  Mainly to allow interrupt thread preemption to work for
  slow interrupts
- Increment V_INTR statistic, so vm.stats.sys.v_intr shows correct value,
  i.e. 'Int' field in systat -vm

Submitted-by: dillon@