Socket: reuse address to resolve the error "address already in use"

On Unix, when we stop the server, and then re-start it right away, we get an
error that the address is already in use:
http://www.softlab.ntua.gr/facilities/documentation/unix/unix-socket-faq/unix-socket-faq-4.html#ss4.1
This means that the sockets that were used by the first incarnation of the
server are still active.

One way to resolve this is to set the socket option SO_REUSEADDR:
http://www.softlab.ntua.gr/facilities/documentation/unix/unix-socket-faq/unix-socket-faq-4.html#ss4.5

Tested on Ubuntu 12.04 LTS