Stock interface to manage reuseAddress, please

In Linux, it's often important to turn on the SO_REUSEADDR flag to make some kinds of code work reliably – particularly tests that start and stop a service repeatedly.

As things are in Jetty, this is a complex process. I propose that the AbstractConnector include a protocol for controlling this, so that jetty XML or Spring XML could easily set it. It would be particularly helpful if the Server object would have a property that asked it to turn on this on all connectors in the server that support the protocol to do so.