Protocol version of Attach API

[Hohensee, Paul]

I'd like to propose an alternative approach, namely, we could add another argument parsing feature on top of the old functionality intact and leave the latter intact. We currently have just a single constant AttachOperation::arg_count_max, which the patch changed from 3 to 4. We could leave it at 3 and add another constant, call it extra_arg_count, and set extra_arg_count to 1 for now. The required number of attachListener args stays at 3, and we add code that checks for up to extra_arg_count arguments. As part of the new code, add a sentinel argument value (all 1s?) to signal end of extra arguments, rather than overloading the null string, since a null string is a valid argument. Opinion(s)?

Thanks,

Paul