- Just use standard mbuf flags in rxd_info
- qset index as arg to txd_encap is gratuitous, just make
  field in if_pkt_info
- comment if_rxd_info and if_pkt_info fields

Interface changes to iflib to accomodate bxe and cxgb
- handle padding in clusters (cxgb)
- handle tx ring doorbells that use some cumulative number
  other than the descriptor index (bxe)
- handle case where driver packet metadata is put in the cluster itself
  (cxgb) and thus the clusters virtual addresses need to be visible to
  the driver
- handle lack of 1 to 1 relationship between rx hardware queue and ifp
  (cxgb 1GigE)