examples/ipsec-secgw: fix usage print

examples/ipsec-secgw: fix usage print

The usage print was not updated when jumbo frames & crypto_dev mask
support was added. Fixing that. Also, the optional arguments were not
properly highlighted in the usage header. This is also fixed.

General cleanup of the usage print was also done to make it look more
cleaner and similar to what is existing in other applications like
l3fwd.

Fixes: bbabfe6e4ee4 ("examples/ipsec_secgw: support jumbo frames")
Fixes: 2c68fe791538 ("examples/ipsec-secgw: add cryptodev mask option")
Fixes: d299106e8e31 ("examples/ipsec-secgw: add IPsec sample application")
Cc: stable@dpdk.org
Signed-off-by: Anoob Joseph <anoob.joseph@caviumnetworks.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>