added a --with-sockethack option that should give us an _enormous_

added a --with-sockethack option that should give us an _enormous_
boost in netbench performance on Linux/SMP if the corresponding 4 line
change is made in the kernel. Note: this is a experimental performance
hack only at the moment. Do _not_ enable it until we find a way to
make it safe in the kernel, which may not be for a while.