This adds a local ifaddrs implementation to be used on Bionic or other C
libraries that don't come with a getifaddrs implementation.

This code was written by Kenneth MacKay and is under a two-clause BSD
license (copyright information in the file headers).
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>