How to distinguish between STREAMS and non-STREAMS

[Guy Harris]

>I_LIST might be particularly fast since if you pass it a NULL pointer
>for the list, it just returns the number of modules on the stream,

Unless, of course, you're not running System V Release 4, in which case
it just returns an error....

I_LOOK also looks only at the stream head, although it does loop through
the table of modules to find the name.