examples/ipsec-secgw: remove redundant string compare

examples/ipsec-secgw: remove redundant string compare

Removing redundant strncmp in parsing long arguments. The getopt library
provides means to identify long options using the "val" field of
structure option. The existing code gets 0 as "val" for all long
arguments and then uses strncmp to figure out which long option was
being referred to. Fixing this.

In addition, the macros and enums used for long arguments have been
renamed and repositioned adhering to the general convention followed in
various other apps, like l3fwd.

Signed-off-by: Anoob Joseph <anoob.joseph@caviumnetworks.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>