Dont' flush write buffers on write error, UNLESS there is no callback handler.

Dont' flush write buffers on write error, UNLESS there is no callback handler.
The callback should check the status return code and take necessary measures,
the disk module will keep the data queued.

I'm pretty sure we don't need to seek on disk files since they are only
open for reading or writing, never both.
    Remove all seek calls
    Remove FD_AT_EOF wierdness
    Remove wierd dread_ctrl->cur_len member