close_fd_read after A_WRITE.

* src/io.c (do_kill_io_backend): Let close_fd do it's work.

* src/io.c (close_fd): Really close the fd, and invoke any close
callback.
(io_iter): Simplified handling of closed files. Simply unlink
them, close_fd has to do the rest of the work.

Rev: src/io.c:1.137