Same root cause as the websocket spin this branch fixes, different symptom, so it
belongs with the InputSink EOF change rather than apart from it.
ReadBinaryUntilTerminator() returns 0 both when nothing has arrived yet and when
the peer is gone, and the multipart loop only exits on finding its terminator - so
a cancelled browser upload left a handler thread spinning forever. That also hangs
web server shutdown, since it joins its handler threads.
Now that InputSink reports EOF, ask it.
(Moved here from misc-correctness-fixes, which will be rebased on top of this.)
Henrik Rydgård
949f773220