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]

config.c

@@ -2490,6 +2490,9 @@ void setup_config_box(struct controlbox *b, int midsession,
 	    ctrl_droplist(s, "Ignores SSH-2 maximum packet size", 'x', 20,
 			  HELPCTX(ssh_bugs_maxpkt2),
 			  sshbug_handler, I(CONF_sshbug_maxpkt2));
+	    ctrl_droplist(s, "Replies to channel requests after channel close",
+                          'q', 20, HELPCTX(ssh_bugs_chanreq),
+			  sshbug_handler, I(CONF_sshbug_chanreq));
 	}
     }
 }