Restate fragment readahead.

Revert special case behavior of set_input_fragment, except that
of passing the supplied maxreadahead to get_input_bytes, which
now takes the maxreadahead parameter in turn.  Finally, carry
maxreadahead on to fill_input_buf, and use it as the max read
length if greater than len.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>