batman-adv: Avoid old nodes disabling multicast optimizations completely

Instead of disabling multicast optimizations mesh-wide once a node with
no multicast optimizations capabilities joins the mesh, do the
following:

Just insert such nodes into the WANT_ALL_IPV4/IPV6 lists. This is
sufficient to avoid multicast packet loss to such unsupportive nodes.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>