arpsweep TODO:
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  - adaptive RTT (a la "ping -A")
  - implement the ignored format specification
  - replace linked list with hash; performance tanks as more IPs to scan
  - fix reporting so -M, -A and -W options actually work
  - compile time BPF selection ("arp and ether dst <ether>" or "arp")
  - OpenBSD, MacOSX portability issues
