dd: pad partial record with spaces in some cases

[jfinkels]

If conv=block,sync command-line arguments are given and there is at
least one partial record read from the input (for example, if the
length of the input is not divisible by the value of the ibs
argument), then output an extra block of cbs spaces.

For example, no extra spaces are printed in this example because the
input is of length 10, a multiple of ibs:

$ printf "012\nabcde\n" \
> | dd ibs=5 cbs=5 conv=block,sync status=noxfer \
> && echo $
012  abcde$
2+0 records in
0+1 records out

But in this example, 5 extra spaces are printed because the length of
the input is not a multiple of ibs:

$ printf "012\nabcdefg\n" \
> | dd ibs=5 cbs=5 conv=block,sync status=noxfer \
> && echo $
012  abcde     $
2+1 records in
0+1 records out
1 truncated record\n

The number of spaces printed is the size of the conversion block,
given by cbs.

(I came to this understanding of the program logic by reverse-engineering the output of GNU dd, so I may be wrong about it.)

[sylvestre]

You have a conflict :) (probably caused by yourself :)