Allow creating connections without authentication

[guelfey]

This is already possible using e.g. NewConn or SessionBusPrivate, which explicitly do not do call Hello or Auth internally at startup.

[itstarsun]

A Conn returned by NewConn still has to call the Auth method manually, otherwise the inWorker will never be called. https://github.com/godbus/dbus/blob/e523abc905595cf17fb0001a7d77eaaddfaa216d/auth.go#L118