MEDIUM: mux: provide the session to the init() and attach() method.

Instead of trying to get the session from the connection, which is not
always there, and of course there could be multiple sessions per connection,
provide it with the init() and attach() methods, so that we know the
session for each outgoing stream.
diff --git a/src/backend.c b/src/backend.c
index 83ae51d..c79302d 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1074,7 +1074,7 @@
 	cs = si_alloc_cs(&s->si[1], conn);
 	if (!cs)
 		goto fail;
-	if (conn_install_mux_be(conn, cs) < 0)
+	if (conn_install_mux_be(conn, cs, s->sess) < 0)
 		goto fail;
 	srv = objt_server(s->target);
 	if (srv && ((s->be->options & PR_O_REUSE_MASK) == PR_O_REUSE_ALWS) &&
@@ -1275,7 +1275,7 @@
 				LIST_DEL(&srv_conn->list);
 				LIST_INIT(&srv_conn->list);
 			}
-			srv_cs = srv_conn->mux->attach(srv_conn);
+			srv_cs = srv_conn->mux->attach(srv_conn, s->sess);
 			if (srv_cs)
 				si_attach_cs(&s->si[1], srv_cs);
 		}
@@ -1324,7 +1324,7 @@
 				conn_free(srv_conn);
 				return SF_ERR_RESOURCE;
 			}
-			if (conn_install_mux_be(srv_conn, srv_cs) < 0)
+			if (conn_install_mux_be(srv_conn, srv_cs, s->sess) < 0)
 				return SF_ERR_INTERNAL;
 			/* If we're doing http-reuse always, and the connection
 			 * is an http2 connection, add it to the available list,