Introduced scripts to listen on multicast group

Cherry-picked from branch 'distributed-multicast'

With the existence of netlogon/linuxclient/server-verify.key, the client will
listen on the multicast address 239.0.11.1 on port 51000 for a message from
the server, which it will verify using the aforementioned key. If the message
is valid, the client executes the message.