ipsec-tools: enable Dead Peer Detection and NAT traversal

* Dead peer detection (DPD) is a method that network devices use to verify
  the current existence and availability of other peer devices.
* NAT traversal (sometimes abbreviated as NAT-T) is a general term for
  techniques that establish and maintain Internet protocol connections
  traversing network address translation (NAT) gateways

Signed-off-by: Ting Liu <b28495@freescale.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>