pktgen: Add missing !flag parameters

o FLOW_SEQ now can be disabled with pgset "flag !FLOW_SEQ"
o FLOW_SEQ and FLOW_RND are antonyms, as it's shown by pktgen_if_show()
o IPSEC now may be disabled

Note, that IPV6 is enabled with dst6/src6 parameters, not with
a flag parameter.

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