Fix bug: when line translation is in effect, reading a raw

Fix bug: when line translation is in effect, reading a raw
carriage-return would cause the end-index of an input buffer to become
zero, indicating that the input channel had reached end-of-file.  This
is a logic error in the design of input buffers; it is fixed by
defining logical end-of-file to be when input-buffer/real-end is zero,
because the latter is unaffected by line translation.