if: Multiple TX queue support step 1 of many; introduce ifaltq subqueue

Put the plain queue information, e.g. queue header and tail, serializer,
packet staging scoreboard and ifnet.if_start schedule netmsg etc. into
its own structure (subqueue).  ifaltq structure could have multiple of
subqueues based on the count that drivers can specify.

Subqueue's enqueue, dequeue, purging and states updating are protected
by the subqueue's serializer, so for hardwares supporting multiple TX
queues, contention on queuing operation could be greatly reduced.

The subqueue is passed to if_start to let the driver know which hardware
TX queue to work on.  Only the related driver's TX queue serializer will
be held, so for hardwares supporting multiple TX queues, contention on
driver's TX queue serializer could be greatly reduced.

Bunch of ifsq_ prefixed functions are added, which is used to perform
various operations on subqueues.  Commonly used ifq_ prefixed functions
are still kept mainly for the drivers which do not support multiple TX
queues (well, these functions also ease the netif/ convertion in this
step :).

All of the pseudo network devices under sys/net are converted to use the
new subqueue operation.  netproto/802_11 is converted too.  igb(4) is
converted to use the new subqueue operation, the rest of the network
drivers are only changed for the if_start interface modification.

For ALTQs which have packet scheduler enabled, only the first subqueue
is used (*).

(*) Whether we should utilize multiple TX queues if ALTQ's packet scheduler
is enabled is quite questionable.  Mainly because hardware's multiple TX
queue packet dequeue mechanism could have negative impact on ALTQ's packet
scheduler's decision.
if: Free subqueue memory upon if_detach