libnetwork: getifaddrs() now returns ipv6 and physical addresses

If available, getifaddrs() will now return additional entries for
different addresses of an interface. This is similiar to how NetBSD does
it.

This allows software such as libuv to construct detailed information
about network interfaces within the system.

Adrien have some concerns about getaddrinfo() wouldn't correctly exclude
IPv6 addresses after this change so a test was added to cover that.

Change-Id: I8c5d3a02b9294d746ca928a285f40344dfd6e3cb
Reviewed-on: https://review.haiku-os.org/c/874
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

tests/kits/net: Move libnetwork-related tests to tests/system/network.