Replace deprecated gethostbyname(3) with getaddrinfo(3)

[linimon]

Summary

The commit just changes definition of getaddr() function which was using gethostbyname(3)
before and uses getaddrinfo(3) instead.

Test Plan

I ran a separate test program to confirm that the replacement produces the same effect.
I didn't test it with arp command itself because I am not that much familiar with it.

[luthramihir708_gmail.com]

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.

[linimon]

Superceded by D21589.