CLEANUP: conn_stream: rename the cs_endpoint's target to "se"

That's the "stream endpoint" pointer. Let's change it now while it's
not much spread. The function __cs_endp_target() wasn't yet renamed
because that will change more globally soon.
diff --git a/src/mux_pt.c b/src/mux_pt.c
index e93c2d6..8750fac 100644
--- a/src/mux_pt.c
+++ b/src/mux_pt.c
@@ -298,7 +298,7 @@
 			TRACE_ERROR("CS allocation failure", PT_EV_STRM_NEW|PT_EV_STRM_END|PT_EV_STRM_ERR, conn);
 			goto fail_free_ctx;
 		}
-		ctx->endp->target = ctx;
+		ctx->endp->se     = ctx;
 		ctx->endp->conn   = conn;
 		se_fl_set(ctx->endp, SE_FL_T_MUX | SE_FL_ORPHAN);