SCP: Tweaks

sending the close channel packet right after the eof seems to make some scp transfers terminate prematurely.

unfortunately, sometimes this behavior is undesirable as it is in this case:

http://www.frostjedi.com/phpbb3/viewtopic.php?f=46&t=29457

hence the $want_reply parameter

also, this commit makes the scp packet length account for the length portion