SourceLayout: pull ARP operations into libeui

SourceLayout: pull ARP operations into libeui

This separates the current code for ARP lookup and encode/decode.
Also makes the code buildable without fatal build errors on
non-supporting systems. A loud run-time error is generated if related
features are attempt use on non-supporting systems.

FUTURE WORK:
  * unit-tests
  * on-demand lookups and logging of MAC/EUI-48 for a client
    similar to how ident is done.
  * on-demand passing MAC/EUI-48 to external_acl_type helpers
  * EUI-64 for IPv6 support.
  * Increased ARP support on other OS.