Core: disabled SO_REUSEADDR on UDP sockets while testing config. On Linux, SO_REUSEADDR allows completely duplicate UDP sockets, so using SO_REUSEADDR when testing configuration results in packets being dropped if there is an existing traffic on the sockets being tested (ticket #2187). While dropped packets are expected with UDP, it is better to avoid this when possible. With this change, SO_REUSEADDR is no longer set on datagram sockets when testing configuration.