Make io_output() return a sane value

Make io_output() return a sane value

Return number of bytes written on success, -1 on error.  In
particular, return zero when nothing was written because the queue was
empty, or because the write slept and got woken up, or because the
write refused to sleep.

Before, it instead returned the number of bytes remaining to be
written when empth_select() failed, when woken up from sleep, or
refusing to sleep.  You couldn't tell from the return value whether
the call made progress writing out the queue.

The current callers don't actually notice the change.