ethdev: add support for device offload capabilities

ethdev: add support for device offload capabilities

1) Make device RX and TX offload capabilities to be returned in the
   rte_eth_dev_info data structure by the function rte_eth_dev_info_get

   The following initial set of RX offload capabilities are defined:
   - VLAN header stripping
   - IPv4 header checksum check
   - UDP checksum check
   - TCP checksum check
   - TCP large receive offload (LRO)

   The following initial set of TX offload capabilities are defined:
   - VLAN header insertion
   - IPv4 header checksum computation
   - UDP checksum computation
   - TCP checksum computation
   - SCTP checksum computation
   - TCP segmentation offload (Transmit Segmentation Offload)
   - UDP segmentation offload

   2) Update the eth_dev_infos_get() function of the igb and ixgbe PMDs
      to return the offload capabilities which are supported by the
      device and that are effectively managed by the driver.

Signed-off-by: Ivan Boule <ivan.boule@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>