Core: disabled SO_REUSEADDR on UDP sockets while testing config.

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.