Fix `disconnect': arrange that we keep track of when we're expecting

Fix `disconnect': arrange that we keep track of when we're expecting
to see the server slam the TCP connection shut (i.e. almost never,
unless it's just sent us an SSH_MSG_DISCONNECT), and treat an
unexpected closure as a non-clean session termination. Previously
any server-initiated connection closure was being treated as a clean
exit, which was a hangover from the good old Telnet-only days.

[originally from svn r5098]
[this svn revision also touched putty-wishlist]