igb: set default Rx/Tx configuration

igb: set default Rx/Tx configuration

Many sample apps use duplicated code to set rte_eth_txconf and rte_eth_rxconf
structures. This patch allows the user to get a default optimal RX/TX configuration
through rte_eth_dev_info get, and still any parameters may be tweaked as wished,
before setting up queues.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Reviewed-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: David Marchand <david.marchand@6wind.com>
[Thomas: split patch]