add support for ARP packets

[mbeckerle]

Standard test files have a small number of ARP packets in them.

Parsing these files (ex: 50m.pcap, 100m.pcap) fails as a result.
(Was reported as an Apache Daffodil ticket: https://issues.apache.org/jira/browse/DAFFODIL-1830)

Best to add this packet type than require users to prepare data which does not have any.

[mbeckerle]

Note that this enhancement (the new packet type) goes not directly in PCAP, but in the ethernetIP schema used by PCAP.