>> And, what should the hard limit be--that even sysctl() can not go above?

>UINT_MAX.

>> (This will become the value of SOMAXCONN such that programs using the
>> macro will gain the higher limits without recompiling.)
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>Leave SOMAXCONN at 128, I say.

But to achieve the underlined part with the extant sockets codebase,
you need to make SOMAXCONN be UINT_MAX.

So then we _need_ yet another option, to set the default upper limit
actually enforced by listen() to something sane.  (where "sane" is
possibly arch-dependent).   Like the knobs Matt suggested.

Is that the wavelength you're on?