examples/ipsec-secgw: add fragment TTL option

examples/ipsec-secgw: add fragment TTL option

Due to fragment loss on highly saturated links and long fragment
lifetime, ipsec-secgw application quickly runs out of free reassembly
buckets. As a result new fragments are being dropped.

Introduce --frag-ttl option which allow user to lower default fragment
lifetime which solves problem of saturated reassembly buckets with high
bandwidth fragmented traffic.

Signed-off-by: Marcin Smoczynski <marcinx.smoczynski@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>
Tested-by: Konstantin Ananyev <konstantin.ananyev@intel.com>