iterate_fd()

I spent (more than) a few hours tracking down why my MPD -> MPlayer setup changed[1] last night. Bisected 2+ years of MPD history twice (using different litmus tests for good/bad) and found a commit that…makes little sense both times. But at least I know where to start now.

[1]MPlayer stopped showing tags in MPD's https output stream, times were bogus, and pausing caused MPlayer to buffer, doubling the silence time (and chewing memory). Second two are due to MPlayer not recognizing audio/ogg as an OGG mimetype (email missing the patch was sent; need to resend with patch yet) and the first was lost on the way to 0.16-alpha1…when replay gain was made an output filter. How that breaks streaming tags, I have no idea (yet).