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.

ssh1connection.c

@@ -950,6 +950,7 @@ static void ssh1_channel_init(struct ssh1_channel *c)
     c->pending_eof = FALSE;
     c->throttling_conn = FALSE;
     c->sc.vt = &ssh1channel_vtable;
+    c->sc.cl = &s->cl;
     c->localid = alloc_channel_id(s->channels, struct ssh1_channel);
     add234(s->channels, c);
 }

ssh2connection.c

@@ -1348,6 +1348,7 @@ static void ssh2_channel_init(struct ssh2_channel *c)
     c->throttle_state = UNTHROTTLED;
     bufchain_init(&c->outbuffer);
     c->sc.vt = &ssh2channel_vtable;
+    c->sc.cl = &s->cl;
     c->localid = alloc_channel_id(s->channels, struct ssh2_channel);
     add234(s->channels, c);
 }

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))