netmap: fix promiscous setting for FreeBSD

This patch set permanent promiscuous mode on FreeBSD. This is the
only mode supported and this is equivalent to do something like:
 ifconfig em0 promisc
Reverting state is not considered as there is no way to guarantee
it will occur. Second reason is that the iface has to be dedicated
to netmap (it even loses address when suricata is started).

A possible choice could be to have a variable like 'force-promiscuous'  in the yaml. By default at no for netmap, it would set promiscuous mode if set to yes. If unset in the YAML we could send a message to the user saying that would be a good idea.