Give SshChannel a pointer to its owning ConnectionLayer.

In my future plans, some SshChannels are going to need to be able to
ask favours from the connection layer as a whole. And an SshChannel is
inextricably tied to an instance of the connection layer, so there's
no real reason _not_ to make the pointer generally available.

sshchan.h

@@ -157,6 +157,7 @@ struct SshChannelVtable {
 
 struct SshChannel {
     const struct SshChannelVtable *vt;
+    ConnectionLayer *cl;
 };
 
 #define sshfwd_write(c, buf, len) ((c)->vt->write(c, buf, len))