Stop target process immediately after execve.

The current way of launching to be debugged processes is kind of racy. They are
started and then get sent a SIGSTOP. This results in the target being stopped
at a more or less arbitrary point. There is explicit support for launching to
be traced processes in os/exec. By setting SysProcAttr.Ptrace we can make the
target process stop immediately after the execve.
The arity of AttachBinary hasn't changed.