- Change kobj methods to take a shared context instead of an iflib context
- expose device_t and if_t in the shared context and eliminate trivial
  accessor functions from iflib
- add tx_flush for poking doorbell register separately from enqueuing tx
  packets
- add common statistics to shared context
- export if_irq
- rename rxd_info if_rxd_info to reflect common public structure
  naming scheme