Fix drbr and altq interaction:

• introduce drbr_needs_enqueue that returns whether the interface/br needs an enqueue operation: returns true if altq is enabled or there are already packets in the ring (as we need to maintain packet order)
• update all drbr consumers
• fix drbr_flush
• avoid using the driver queue (IFQ_DRV_*) in the altq case as the multiqueue consumer does not provide enough protection, serialize altq interaction with the main queue lock
• make drbr_dequeue_cond work with altq

Discussed with: kmacy, yongari, jfv
MFC after: 4 weeks