sd-network: IPv4 link-local support [v2]

Implements IPv4LL with respect to RFC 3927
(http://tools.ietf.org/rfc/rfc3927.txt) and integrates it
with networkd. Majority of the IPv4LL state machine is
taken from avahi (http://avahi.org/) project's autoip.

IPv4LL can be enabled by IPv4LL=yes under [Network]
section of .network file.

IPv4LL works independent of DHCP but if DHCP lease is
aquired, then LL address will be dropped.

[tomegun: removed a trailing newline and a compiler warning]

src/libsystemd/sd-rtnl/rtnl-message.c

@@ -85,6 +85,20 @@ int sd_rtnl_message_route_set_dst_prefixlen(sd_rtnl_message *m, unsigned char pr
         return 0;
 }
 
+int sd_rtnl_message_route_set_scope(sd_rtnl_message *m, unsigned char scope) {
+        struct rtmsg *rtm;
+
+        assert_return(m, -EINVAL);
+        assert_return(m->hdr, -EINVAL);
+        assert_return(rtnl_message_type_is_route(m->hdr->nlmsg_type), -EINVAL);
+
+        rtm = NLMSG_DATA(m->hdr);
+
+        rtm->rtm_scope = scope;
+
+        return 0;
+}
+
 int sd_rtnl_message_new_route(sd_rtnl *rtnl, sd_rtnl_message **ret,
                               uint16_t nlmsg_type, unsigned char rtm_family) {
         struct rtmsg *rtm;