pcap_set_promisc(3) on FreeBSD

The text 41f141c2039d, as FreeBSD 8.4-RELEASE, FreeBSD 8.3-RELEASE, FreeBSD 8.2-RELEASE, FreeBSD 8.1-RELEASE, FreeBSD 8.0-RELEASE shipped it.

set promiscuous mode for a not-yet-activated capture handle