For mac80211 devices we need to clean up monitor mode on exit.

For mac80211 devices we need to clean up monitor mode on exit.

Just as with non-mac80211 devices, the stuff we do to enter monitor mode
doesn't automatically get cleaned up when the capture device is closed,
so we need to arrange that pcap_close_all() be called when we exit, to
close all the pcap_t's and turn off monitor mode.