agent: Use getnameinfo() instead of inet_ntop() to stringify addresses

agent: Use getnameinfo() instead of inet_ntop() to stringify addresses

inet_ntop() drops IPv6 link-local scope IDs on the floor, which means
libnice doesn’t generate any IPv6 candidates for link-local connections,
meaning that it will theoretically fail to negotiate a connection on an
IPv6-only network where the two peers are local.

Fix that by using getnameinfo() to convert sockaddrs to strings, which
preserves the scope ID.

There are no portability concerns — getnameinfo() is POSIX.1-2001, and
isn’t used on Windows (although it is supported, surprisingly).