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.
Maybe splitting it up into:

    proctl.Attach(pid int) (*DebuggedProcess, error)
    proctl.Launch(cmd []string) (*DebuggedProcess, error)

makes sense?