channel_io_status. Don't call channel_read_state_close on error;
leave that up to the caller.
(channel_io_start_read): Check that file->fd is valid.
(channel_io_write, channel_io_flush): Return an enum
channel_io_status. Don't call channel_write_state_close on error;
leave that up to the caller.
(channel_io_stop_write): Return CHANNEL_IO_EOF if all data has
been written.

* src/channel_io.h (enum channel_io_status): New enum.

Rev: src/channel_io.c:1.1.2.4
Rev: src/channel_io.h:1.1.2.2