Implement this year's consensus on CHANNEL_FAILURE vs CHANNEL_CLOSE.

We now expect that after the server has sent us CHANNEL_CLOSE, we
should not expect to see any replies to our outstanding channel
requests, and conversely after we have sent CHANNEL_CLOSE we avoid
sending any reply to channel requests from the server. This was the
consensus among implementors discussing the problem on ietf-ssh in
April 2014.

To cope with current OpenSSH's (and perhaps other servers we don't
know about yet) willingness to send request replies after
CHANNEL_CLOSE, I introduce a bug-compatibility flag which is detected
for every OpenSSH version up to and including the current 6.6 - but
not beyond, since https://bugzilla.mindrot.org/show_bug.cgi?id=1818
promises that 6.7 will also implement the new consensus behaviour.

[originally from svn r10200]

putty.h

@@ -838,6 +838,7 @@ void cleanup_exit(int);
     X(INT, NONE, sshbug_maxpkt2) \
     X(INT, NONE, sshbug_ignore2) \
     X(INT, NONE, sshbug_winadj) \
+    X(INT, NONE, sshbug_chanreq) \
     /*                                                                \
      * ssh_simple means that we promise never to open any channel     \
      * other than the main one, which means it can safely use a very  \