[NETFILTER]: merge ipt_owner/ip6t_owner in xt_owner

xt_owner merges ipt_owner and ip6t_owner, and adds a flag to match
on socket (non-)existence.

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>