pcap_can_set_rfmon(3) on FreeBSD

The text 8d01001174a5, as FreeBSD 8.4-RELEASE, FreeBSD 8.3-RELEASE, FreeBSD 8.2-RELEASE, FreeBSD 8.1-RELEASE, FreeBSD 8.0-RELEASE shipped it.

check whether monitor mode can be set for a not-yet-activated capture handle