Change variable arp_op to arp_opcode to avoid name collision with netinet/ether.h

The detail problem description could be found [here on trema-dev](https://groups.google.com/forum/#!topic/trema-dev/YaZJYqUwOCw)

A test example could be found here:
[Makefile] (https://gist.github.com/rascov/9949933)
[arp_opcode_test.c] (https://gist.github.com/rascov/9949949)
After updating trema-edge to this version, we no longer need to #undef arp_op anymore while using netinet/ether.h