examples/ipsec-secgw: allow disabling some Rx/Tx offloads

examples/ipsec-secgw: allow disabling some Rx/Tx offloads

Right now ipsec-secgw always enables TX offloads
(DEV_TX_OFFLOAD_MULTI_SEGS, DEV_TX_OFFLOAD_SECURITY),
even when they are not requested by the config.
That causes many PMD to choose full-featured TX function,
which in many cases is much slower then one without offloads.
That patch adds ability for the user to disable unneeded HW offloads.
If DEV_TX_OFFLOAD_IPV4_CKSUM is disabled by user, then
SW version of ip cksum calculation is used.
That allows to use vector TX function, when inline-ipsec is not
requested.

Signed-off-by: Remy Horton <remy.horton@intel.com>
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>