app/testpmd: add commands to test new offload API

Add following testpmd run-time commands to support test of
new Rx offload API:
show port <port_id> rx_offload capabilities
show port <port_id> rx_offload configuration
port config <port_id> rx_offload <offload> on|off
port <port_id> rxq <queue_id> rx_offload <offload> on|off
Above last 2 commands should be run when the port is stopped.
And <offload> can be one of "vlan_strip", "ipv4_cksum", ...

Add following testpmd run-time commands to support test of
new Tx offload API:
show port <port_id> tx_offload capabilities
show port <port_id> tx_offload configuration
port config <port_id> tx_offload <offload> on|off
port <port_id> txq <queue_id> tx_offload <offload> on|off
Above last 2 commands should be run when the port is stopped.
And <offload> can be one of "vlan_insert", "udp_cksum", ...

Signed-off-by: Wei Dai <wei.dai@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>