Support direct control of the server listener for easier integration

[csweichel]

Integrating a chisel server into an existing component is somewhat tricky right now, because there's no direct control over the net.Listener the chisel server uses. E.g. when trying to integrate chisel with cmux that becomes a problem.

This PR enables direct control over the chisel server's net.Listener. Using this change we were able to integrate chisel using cmux.