pcap_can_set_rfmon(3) on FreeBSD

The text 993e042f5efc, as FreeBSD 12.1-RELEASE, FreeBSD 12.0-RELEASE, FreeBSD 11.3-RELEASE, FreeBSD 11.2-RELEASE, FreeBSD 11.1-RELEASE shipped it.

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