Make Nmap (nsock) more tolerant of disappearing interfaces

We've received some complaints of a scan halting with:

pcap_next_ex() fatal error while reading from pcap: The interface disappeared (error code ERROR_DEVICE_REMOVED/STATUS_DEVICE_REMOVED)

It may just be the network interface temporarily disappearing (like disconnecting from wifi or switching to ethernet when plugging into a docking station). So we may want to try and handle this error more softly, like waiting a bit and trying again, and/or maybe recheck the routing table and switching to the new appropriate interface.