Improve vnet(4) to work better in busy conditions.

Improve vnet(4) to work better in busy conditions.
No longer limit the ifq size to a low number, increase the slots on the
DMA Ring a bit and abstract the VNET buffer size into a define.
Enqueue packets on the ring but mark the initial packet ready at the end.
This way the other ldom is not able to rush ahead and overconsume packets.
The dring indexes are passed between ldoms and can get out of sync with
causes the TX ring to stall.
Tested by myself and jan@
OK kettenis@ jan@ kn@