Set monitor mode for interface

"-I/--monitor-mode" option is used to put network interface in "monitor mode" through pcap_set_rfmon API (code is here):

......
if (Iflag) {
    status = pcap_set_rfmon(pc, 1);
    if (status != 0)
        error("%s: Can't set monitor mode: %s",
            device, pcap_statustostr(status));
}
......

Definitely, the interface should support "monitor mode" first, otherwise, following error message will be outputted:

# tcpdump -I
tcpdump: enp0s3: That device doesn't support monitor mode