ofproto: Add support for matching IP addresses in ARP header (OpenFlow 1.0)

The OpenFlow 1.0 specification supports matching the IP address and
opcode in ARP messages.  The datapath already supports this, so this
commit merely exposes that through the OpenFlow module.

NOTE: OVS at this point is not wire-compatible with OpenFlow 1.0
until the final commit in this OpenFlow 1.0 set.