Add an additional queue which will be "owned by the driver". This allows to

rig a PREPEND macro for ALTQ as the POLL/DEQUEUE semantic is very bad in
terms of locking. We make this a full functional queue to allow "bulk
dequeue" which will further reduce the locking overhead (for non-altq
enabled devices). Drivers will access this via the following macros, which
will show up in <net/if_var.h> once we expose ALTQ to the build:

IFQ_DRV_DEQUEUE(ifq, m)	- takes a mbuf off the queue (driver queue first)
IFQ_DRV_PREPEND(ifq, m)	- pushes a mbuf back to the driver queue
IFQ_DRV_PURGE(ifq)	- drops all packets in both queues
IFQ_DRV_IS_EMPTY(ifq)	- checks for pending mbufs in either queue

One has to make sure that the first three are protected by a driver mutex.
At the moment most network drivers still require Giant, so this is not an
issue. Even those that have thier own mutex usually hold it in if_start and
the like, so this requirement is almost always satisfied.

This evolved from a discussion with Andrew Gallatin.

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