Use unix socket for local multicast loop

Instead of rely on multicast loop functionality of kernel, we now use
unix socket created by socketpair to deliver multicast messages to
local node. This handles problems with improperly configured local
firewall. So if output/input to/from ethernet interface is blocked, node
is still able to create single node membership.

Dark side of the patch is fact, that membership is always created, so
"Totem is unable to form a cluster..." will never appear (same applies
to continuous_gather key).

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>