ifq/staging: Initial implementation of IFQ packet staging mechanism

ifq/staging: Initial implementation of IFQ packet staging mechanism

The packets enqueued into IFQ are staged to a certain amount before the
ifnet's if_start is called.  In this way, the driver could avoid writing
to hardware registers upon every packet, instead, hardware registers
could be written when certain amount of packets are put onto hardware
TX ring.  The measurement on several modern NICs (emx(4), igb(4), bnx(4),
bge(4), jme(4)) shows that the hardware registers writing aggregation
could save ~20% CPU time when 18bytes UDP datagrams are transmitted at
1.48Mpps.

IFQ packets staging is performed for direct ifnet's if_start calling,
i.e. ifq_try_ifstart()

IFQ packets staging will be stopped upon any of the following conditions:
- If the count of packets enqueued on the current CPU is great than or
  equal to ifq_stage_cntmax.
- If the total length of packets enqueued on the current CPU is great
  than or equal to the hardware's MTU - max_protohdr.  max_protohdr is
  cut from the hardware's MTU mainly bacause a full TCP segment's size
  is usually less than hardware's MTU.
- if_start interlock (if_snd.altq_started) is not released.
- The if_start_rollup(), which is registered as low priority netisr
  rollup function, is called; probably because no more work is pending
  for netisr.

Currently IFQ packet staging is only performed in netisr threads.

Inspired-by: Luigi Rizzo's netmap paper
    (http://info.iet.unipi.it/~luigi/netmap/)
Also-Suggested-by: dillon@