ptyfwd,run: process remaining outputs in IO event sources

ptyfwd,run: process remaining outputs in IO event sources

This partially reverts 12807b5a49d1fe60434d473afe11ff81a4c92306.

Otherwise, reading or writing a fd in on_exit_event() handler may return
EBUSY, and the event loop may finish with -ELOOP.

Also, this makes drained() returns true if the PTY forwarder is already
disconnected, for safety. Hence, it is not necessary to re-introduce
pty_forward_is_done().