adjust ftruncate() logic to handle servers that reorder requests.

adjust ftruncate() logic to handle servers that reorder requests.

sftp/scp will ftruncate the destination file after a transfer completes,
to deal with the case where a longer destination file already existed.
We tracked the highest contiguous block transferred to deal with this
case, but our naive tracking doesn't deal with servers that reorder
requests - a misfeature strictly permitted by the protocol but seldom
implemented.

Adjust the logic to ftruncate() at the highest absolute block received
when the transfer is successful. feedback deraadt@ ok markus@

prompted by https://github.com/openssh/openssh-portable/commit/9b733#commitcomment-110679778