'ristsender' joins a multicast address even if there's no RIST receiver on the other side
When converting multicast to RIST, ristsender opens the input socket and joins the multicast address automatically, even if no handshake from any ristreceiver is performed. It'd be useful to have an option to tell 'ristsender' to only start joining the multicast address after a call from ristreceiver is received. So actually 'halt' the ristsender process until a handshake with a client is achieved.