Reduce ifnet.if_serializer contention on output path:

Reduce ifnet.if_serializer contention on output path:
- Push ifnet.if_serializer holding down into each ifnet.if_output implementation
- Add a serializer into ifaltq, which is used to protect send queue instead of
  its parent's if_serializer.  This change has following implication:
  o  On output path, enqueueing packets and calling ifnet.if_start are decoupled
  o  In device drivers, poll->dev_encap_ok->dequeue operation sequence is no
     longer safe, instead dequeue->dev_encap_fail->prepend should be used
  This serializer will be held by using lwkt_serialize_adaptive_enter()
- Add altq_started field into ifaltq, which is used to interlock the calling
  of its parent's if_start, to reduce ifnet.if_serializer contention.
  if_devstart(), a helper function which utilizes ifaltq.altq_started, is added
  to reduce code duplication in ethernet device drivers.
- Add if_cpuid into ifnet.  This field indicates on which CPU device driver's
  interrupt will happen.
- Add ifq_dispatch().  This function will try to hold ifnet.if_serializer in
  order to call ifnet.if_start.  If this attempt fails, this function will
  schedule ifnet.if_start to be called on CPU located by ifnet.if_start_cpuid
  if_start_nmsg, which is per-CPU netmsg, is added to ifnet to facilitate
  ifnet.if_start scheduling.  ifq_dispatch() is called by ether_output_frame()
  currently
- Use ifq_classic_ functions, if altq is not enabled
- Fix various device drivers bugs in their if_start implementation
- Add ktr for ifq classic enqueue and dequeue
- Add ktr for ifnet.if_start