examples/ipsec-secgw: support configuration file

examples/ipsec-secgw: support configuration file

This patch adds the configuration file support to ipsec_secgw
sample application. Instead of hard-coded rules, the users can
specify their own SP, SA, and routing rules in the configuration
file. A command line option "-f" is added to pass the
configuration file location to the application.

Configuration item formats:

SP rule format:
sp <ip_ver> <dir> esp <action> <priority> <src_ip> <dst_ip> \
<proto> <sport> <dport>

SA rule format:
sa <dir> <spi> <cipher_algo> <cipher_key> <auth_algo> <auth_key> \
<mode> <src_ip> <dst_ip>

Routing rule format:
rt <ip_ver> <src_ip> <dst_ip> <port>

Signed-off-by: Fan Zhang <roy.fan.zhang@intel.com>
Acked-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>