[Patch] Fix seekoff/pos and sputbackc testcases

[Benjamin Kosnik]

>the below provides correct unbuffered seekoff/pos testcases and
>fixes the existing buffered ones together with those for sputbackc
>(both unbuffered and buffered).

Looks excellent, thanks. Please check these in.

>Now all the buffered testcases are identical to the unbuffered.

Great news!

>In the near future I'd like to provide some more seek testcases
>for pure in and out.

Sounds good.

best,
benjamin