Set TTL to 255 for outgoing IPv4 unicast packets

Set TTL to 255 for outgoing IPv4 unicast packets

1) We SHOULD do this according to the RFC 6762 (Section 11, "Source
   Address Check").
2) We already do it for IPv6 unicast and both multicast interfaces
3) When receiving IPv4 packet we ignore it if TTL doesn't equal 255

In other words this is a requirement for working unicast communication
between 2 umdns instances (which doesn't seem to work yet though).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: John Crispin <john@phrozen.org>