1. use getopt(3) to parse options
2. introduce -4/-6 flags to enable ipv4/6
3. introduce multicast_addr6 for ipv6 multicasting with hops/ttl
4. clean receiver_process() to receive from both ip4/6

Made corrections as suggested.

1. Updating D21589: arp.c: use getaddrinfo(3) instead of gethostbyname(3) #
2. Enter a brief description of the changes included in this update.
3. The first line is used as subject, next lines as comment. #
4. If you intended to create a new revision, use:
5. $ arc diff --create

1. Updating D21589: arp.c: use getaddrinfo(3) instead of gethostbyname(3) #
2. Enter a brief description of the changes included in this update.
3. The first line is used as subject, next lines as comment. #
4. If you intended to create a new revision, use:
5. $ arc diff --create

I have made some corrections in the above code.
It can be found on https://reviews.freebsd.org/D21589
Apologies for creating a new review request. I made this phabricator review through arc command line and got a little jumbled while trying to update it.
I will be careful from next time.