pcap_get_selectable_fd(3) on FreeBSD

The text 6ddc1d71ae77, as FreeBSD 8.4-RELEASE, FreeBSD 8.3-RELEASE, FreeBSD 8.2-RELEASE, FreeBSD 8.1-RELEASE, FreeBSD 8.0-RELEASE shipped it.

get a file descriptor on which a select() can be done for a live capture