Fix pcap_can_set_rfmon and add pcap_set_rfmon

`pcap_can_set_rfmon` has only one argument (see: http://www.tcpdump.org/manpages/pcap_can_set_rfmon.3pcap.html)
I've also added `pcap_set_rfmon` to actually set monitoring mode (see: http://www.tcpdump.org/manpages/pcap_set_rfmon.3pcap.html)