ip address: Export ip_linkaddr_list

ip address: Export ip_linkaddr_list

ipaddr_list_flush_or_save generates a list of nlmsg's for links and
optionally for addresses. Move the code into ip_linkaddr_list and
export it along with the supporting infrastructure.

API to use this function is:
        struct nlmsg_chain linfo = { NULL, NULL};
        struct nlmsg_chain ainfo = { NULL, NULL};

        ip_linkaddr_list(family, filter_req, &linfo, &ainfo);

        ... error checking and code looping over linfo/ainfo ...

        free_nlmsg_chain(&linfo);
        free_nlmsg_chain(&ainfo);

Signed-off-by: David Ahern <dsahern@gmail.com>