Handle AppleTalk devices

Fixed a bug that prevented Nmap from finding any interfaces when one
of them had the type ARP_HDR_APPLETALK; this was the case for
AppleTalk interfaces. However, This support is not complete
since AppleTalk interfaces use different size hardware addresses
than Ethernet. Nmap IP level scans should work without any problem,
please refer to the '--send-ip' switch and to the following thread:
http://seclists.org/nmap-dev/2013/q1/214
This bug was reported by Steven Gregory Johnson on IRC.