[RFC][BZ #16046] dl_iterate_phdr should not expose internal stub.

[Roland McGrath]

16046 suggests that the status quo today is a regression from 2.18.  If
that's true, then I think it does need to get fixed in 2.19 so that the
user-visible behavior does not differ (except in the intended ways for
static executables).  Further cleanup could wait until after 2.19, but if
it's straightforward then I'd be just as happy to see it done right away.
(The 2.19 release manager may feel differently.)