pktgen: Add behaviour flags macro to generate flags/names

PKT_FALGS macro will be used to add package behavior names definitions
to simplify the code that prints/reads pkg flags.
Sorted the array in order of printing the flags in pktgen_if_show()
Note: Renamed IPSEC_ON => IPSEC for simplicity.

No visible behavior change expected.

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>