unix/moduselect: Implement ipoll() method with no-allocation policy.

ipoll() allows to poll streams without allocating any memory: this method
returns an iterator (a poll object itself), and the iterator yields
preallocated "callee-owned tuple" with polling results for each active
stream. The only operation a caller is allowed to do with this tuple is
extracting values from it (storing the tuple as a whole somewhere is
not allowed).